In the Boot Rant thread, we entertained the possibility of pouring silicone or equivalent material into a boot to make it fit.
I mentioned I hadn't tried it. I hope no one else did.
Having experimented a bit with a different way of creating a form-fitting boot (tape wrap), I can now say this would be a very bad idea.
If a stiff boot fits your foot too well, you can not get the foot in and out of the boot.
The only way to get your foot back out would be to cut the boot.
It is perhaps remarkable that current generation low tech boots have been able to provide adequate stiffness, yet are possible to put on and take off. This is one of the central problems of boot and shoe making.
It is probably a good part of the reason that current generation skate boots don't fit better than they do.
We can imagine form fitting boot technologies that might work - e.g., use a material which stiffens the boot when an electric current runs through it. Then you slip the boot on like a sock, it stretches to match your foot. You turn off the current, and you have an instant boot. Of course you also need to be able to flex your foot enough to move some, which adds a little complexity.
There are also ski boots which expand front, and to both sides, when the closure mechanism is released.
But for now: Do not pour silicone into your boots!