Uhm....we need to stop trying to fix issues that aren't going to be fixed by making soles that won't work and out of weird things that will potentially get the skater injured. Also, replacing boots are quite expensive and one of the more expensive things for a skater to replace depending on their financial situation and how often they need boots based on their skating level. Many elite singles skates can go through at least 3 pairs of skates a competitive year. Many carry two pairs at a time and alternate b/t them. And lastly, there are still boot makers, esp boots that are old school and customs that do use wood or wood byproducts.
I didn't really originate any of these ideas. They are fairly widely talked about in the sports community, including in the skating community. It's really nothing but geometry and basic intuitive physics.
There is no reason they won't work - and I have used them. (Of course cardboard isn't very durable against sheer - so it would be a rather temporary fix.) In particular, cardboard has roughly similar compression properties to the hardened leather that many skate boot midsoles are made from. If she isn't using an insole, she is likely placing her socked feet on the midsole.
But if the foot is loose inside the boot, you to some extent loose control of the motions of your foot, which is likely to contribute to injury. That's true in any shoe - but in skating boots, your feet are elevated from the surface, and balances on a relatively thin piece of metal, so you need to limit slippage more than most sports. In addition, the elevation means that the surface forces are magnified by the leverage that height can create, which makes it easier for the foot to slip inside the boot.
My original Klingbeil insoles were made from leather - though it wasn't hardened. Leather can be a very durable material, and I wish I had kept them, even if I would have needed to modify them to fit too. But I let a skate tech who had no idea what he was doing replace them with something that didn't work any better.
I and many others have tried a variety of tapes to modify insoles. I started with duct tape. It worked well for at most a few days, sometimes for only a few hours. But it shifts around, and I think it changes shape as the adhesives work out from under the tape. Moleskin works well for a fairly short time, and has been widely used for the purpose by many people. I settled on cloth first aid tape (sometimes called athletic tape, or coach tape - especially high quality brands like Johnson & Johnson), because it is the most stable I have found. Cutting oneself a custom insole out of camping mattress foam works a little better - but it only works a few months before matting down, and cutting a 3D shape out of foam takes some practice - it makes sense to figure out first what shape you need to equalize pressure (if that is what you want) using tape.
Typical insoles are made from foams, with various properties. I did not suggest Poron, which is one of the best in the sense that it molds under pressure to the feet, and is widely used in many types of athletic shoe - but it is expensive and it too has a relatively short lifetime. E.g., in typical running shoes, when used by people who run daily, they are often replaced 3-4 times/year. I have used the foam from camping mattresses a lot because it is cheap, and somewhat more durable than other foams I have tried, and because it is skin-safe. (Some urethane foams are much more durable, but I haven't found any that are skin-safe. They are widely used in medical orthotics, by interposing one or more other skin-safe layers.)
I've tried a number of commercial insole products, from companies like Superfeet. None of them worked well for me, though that might be because they didn't happen to have the right shape or physical characteristics for my needs.
The most likely thing to injure someone is if a boot doesn't fit - either because the foot moves around inside the boot, or because it impinges on some part of it. E.g., I used to have a lot of sprained ankles, probably in part because I wore shoes & boots that didn't fit my relatively narrow heels. A lot of that was while backpacking. (Most people would say the heel should lift off the bottom of the boot while backpacking - but other types of motion are not good.) I used very high quality hiking boots, but it hadn't occurred to me to modify the insoles. I'm fairly certain that doing so would have solved a lot of that problem - though I also hadn't learned yet how to fall safely, or prevent falls without using excessive muscle tension. But you can often make footwear fit by adding a carefully crafted or modified insole. Not doing that because you are worried it isn't the official insole the boot comes with with, or isn't produced by a commercial company or medical expert (in which case it is called an orthotic) makes no sense at all to me.
As far as cost: It is true that skate boot costs have gone up a lot since Covid hit. Especially custom skate boot costs. But: Assume a fairly serious competitive skater pays at least $100/week for two or more lessons each week. (Some take a lot more than that.) That's $5200/year. If they use a good skate tech to sharpen their boots, they typically pay $10-$22, every week or two. That's $260 or more/year. And they likely replace their blades - often $300-$1000 every year or two. (I sharpen my own, using hand tools that greatly extend the life of blades, because to some extent the metal is reshaped - sometimes just straightened - instead of ground away - but I have come to the conclusion that relatively few people are willing to learn.) If they attend 4-8 freestyle sessions a week at, say $10-$15 each, that's $40-$120/week, or $2080-$6240 / year. They probably pay $50-$100 / year or more in club club memberships (i used to have to join several clubs to find enough ice time, though I was never all that serious). And they might pay $50-200 a year or more to enter competitions, $100-$200 / year in test fees. Many also pay fees for off-ice training and gym fees. Oh. I forgot about costumes. I've been a bit shocked to hear girls and ladies talk about $1000 for a skating outfit, that is only used for one competition. (Then again, in the ballroom dance community, $3000 is quite common.) Most serious skaters also pay their coaches to come to tests and competitions with them - might well be up to a few hundred $, though that is complicated, because the coach's fees are often split by several skaters.
Add to that all the other classes they might take off-ice, all nominally taken to improve their skating. Ballet. Gymnasitics. Yoga or stretch class. A surprising number are also home schooled with the help of paid tutors, to give them more time to skate and do all these things. I don't know how to estimate those costs. Some skaters have sport-related medical costs as well.
But I think the biggest expenses, that most people don't count, are travel expenses. Traveling to and from the rinks (I've been a bit surprised how far some people travel to take lessons), to and from their favored skate tech for sharpening, to and from tests and competitions, and to all those other classes. The IRS estimates the total costs of driving (gas is only a fraction of this) at $0.67/mile. I'd bet the typical serious skater drives (or is driven, since many are young kids) a few hundred miles / week. Call it 300. That's over $10000/year. (I've known skaters who traveled a lot more than that. Hard to imagine. I had a coach who traveled over 100 miles/day. And another who bought a townhouse in a ritzy neighborhood close to a rink - could we call the cost of the townhouse a skating expense?) Some of the competitions may involve out of town hotel stays too. And, unless Safesport rules have changed again, the coach has to have a separate room. Some skaters have air travel costs as well. (I think travel expenses are actually worse for hockey kids who are on travel teams. They and their families typically go out of town every couple weeks or so.)
And a few kids with busy parents are driven by paid chauffeurs, or someone may be paid to travel with them to hotels, especially now that USFS doesn't let a kid stay in a room with their coach. Of course, one typically has to put up the coach in a hotel room too.
If you need 3 skates / year, it might be because you don't build custom insoles, and pad boots otherwise to equalize pressure inside, or because you take care of them badly - e.g., you leave them inside skate bags, or inside the trunk of your car, etc. But to some extent skates break down because there is empty or low pressure space inside to break down into.
OTOH, I admit that people doing the highest jumps probably place a lot of stress on their boots. Maybe a few break down that many pairs in spite of doing everything right. But it isn't all that common for most of the skaters I have met to need 3 skate pairs / year. Two is somewhat common, but most need less. (Of course there is a growth stage, where kids outgrow skates ridiculously fast. And pregnancy related foot swelling. And some skaters have two complete pairs of skates, in case one breaks or is lost in travel, or they frequently ship one pair back and forth to their skate tech. And that's not counting the skaters who use different skates for different disciplines, or who paint two pair of skates differently for different programs.)
And I do admit that many serious skaters, if they didn't skate, would do something just as expensive. In which case these cost estimates are irrelevant.
How many hours per week have you been skating, and do you alternate your two pairs of skates or use one and keep the others in reserve? They're breaking down after four years, in either case. For an adult, the stiffness rating leans more towards the skater's size than level of ability. I'm your height, weigh about 110, and most of my shoes are size 4 (North American size). Age is forcing me to scale back from triples to double/double combinations in jumps and doubles in pairs throws, and like you I have one weaker ankle from an old fracture. I have a pair of Riedell Silver Star boots on order and currently am wearing Riedell 2010 Fusions. Last year I was wearing Risport RF1 Elite boots. Those all have a 90 stiffness rating, whereas your Motions have a 70 rating (although stiffness is not consistently numbered across all boot brands).
