I know most people here don't like the idea doing this themselves - but I have sometimes used cloth athletic tape to modify insole shapes, especially at the heel, where mine tend to be narrow. The idea was to create what I arbitrarily call "wings" - tape extensions that go on the side or back of the heel to fill the space. You can also stick a good quality "moleskin" to the sides of the heels of the boots. (Obviously, you don't want anything sticky touching your foot because it wouldn't last after you took off your boots. My wings stuck to the bottom of the insole.) Also you can put tape under the back of the heel, to try to push the heel up into a smaller part of the boot. I have sometimes also sculpted my own insoles out of foam, in a full 3 dimensional sense, using a sharp heavy duty scissors. But the foam I used - from a foam camping mat - doesn't retain its shape for more than a few months at most, and often less, before becoming squished down. I chose it because it was cheap and skin-safe - many foams aren't either.
But you say your boots are broken down. In which case anything you do now would likely be very temporary. But maybe worth it while you wait for your new boots to arrive.
You can also buy many types of heel inserts in stores - but it is virtually impossible to determine before you buy them whether they will fit your precise needs. I never found any that fit me.
In principle a good orthotist can take good enough measurements to get inserts made that may work well instead of or in addition to your insoles. A few weeks ago I spoke to an orthotist who told me that to do that, they need a "blank prescription" from a podiatrist. But I don't know if that is true everywhere in the world. Even within the U.S.A., the laws on what each type of medical practitioner can do vary state-to-state. The blank prescription allows the orthotist is to figure out what is needed, rather than follow what the podiatrist specifies. She claimed that orthotists have more training to design inserts than most podiatrists, but I don't know enough to verify that.
If you intend to use inserts, tell your fitter, because it may affect how they fit you.
BTW, there are strong limits on how far you can stretch toeboxes. Maybe about 1/6 - 1/3 inch, though that may be out of date? Probably depends on how stiff the boots are, and what they are made of. And the sole probably can't be stretched at all, any place in the boot - only the upper, starting a bit above the sole. Unless you have unusual soles that are made of heat moldable materials. (The insoles I tried like that didn't work well for me.)
There is something I don't understand: A lot of hockey boots fit me fairly well, unmodified. But so far, no figure skate boots. Because my toes are too wide, my heels too narrow, and my ankles too big. But the fact that many hockey boots fit implies my feet can't be all that unusual, at least for men. (BTW, if you are a lady, consider trying men's boots. They sometimes have wider toes. But you might need to change the color if you don't like black.) Unfortunately, hockey boots and figure skating boots are not easily interchangeable.
