I already tried that type of thin heat-moldable insole, which warp to fit the bottom of the foot. I tried a couple of brands, including, I think, ones from Superfeet, specifically designed for skates.
BTW, they don't really need to be so terribly thin if you remove the old insoles, or replace them with something thinner, cut to shape. And it makes sense to me, if you are willing to do what it takes to make them fit
to start with boots that are slightly too large, because it is easier to fill space then to stretch to fit. (I've tried stretch fits too - the boot eventually unstretches, so you have to keep redoing the stretch, especially with modern thick boots.)
Those thin heat-moldable insoles didn't work well for me. Specifically, one brand rocked around inside the skate. The other brand fit tight, but exerted sufficient pressure on the sides of the boots to not rock very much, but the boots eventually changed their shape from that pressure, expanding outwards, so the fit became very loose. It is better to have the insole mold volumetrically - i.e., compress where needed to fit the foot.
Some ski shops use cork insoles, which are heated before foot insertion. Some people on this forum have used cork too. But I assume cork isn't very durable, and that it takes a fair amount of expertise and equipment - especially since it molds at a fairly high temperature e.g., https://www.pelsupply.com/products/productline/3/83
gives 200 - 275 degrees F for "Birko-cork", which is one such product. That sounds too hot for skin contact - I'm not sure what they do to make it safe. (Maybe it helps that cork, has a low heat capacity - so it is a little like walking on hot bubble-filled volcanic rock?)
What I have been most happy with so far is cutting a piece of foam (I currently use a high quality closed cell foam carpet pad) to 3D shape. It doesn't just go under the foot - I wrap it around anywhere there is excess space or insufficient contact pressure. I start with boots that are a little large, which eliminates the It's very cheap, and I can experiment, and I have by now got it down to a few minutes.
I am fortunate not to have the type of problem, that some skaters have, in which the arches collapse asymmetrically - i.e., if I place my edge down on the ice at a given depth, it stays at that edge depth when I put my full weight on it, which works well for figure skating. So all I need to do is to make the feet feel the same pressure everywhere, and everything will be fine. (Except, of course, that for various health reasons one should avoid sideways and front pressure on the toes, which is why people making foot casts to use in making lasts for custom shoes often use toe caps while making the cast. Also, you need extra space ahead of and behind the area above the ankle, so you can flex and point the ankle.)
But I'm diabetic. My feet are still sensitive to places that have more or less pressure on them, but it is quite common to get a condition called "diabetic feet", or "insensitive feet", in which the feet are numb and insensitive to pressure. If I stop being able to feel pressure, I won't be able to use that sensation to create a good fit.
I am also considering this as an issue for other people, who, when I describe the idea of cutting foam to shape in a 3 dimensional way to fit, say it is much too difficult. I figured out how to do this over a period of years, starting with a more primitive method, adding tape and adhesive foam to thin insoles, cut to the same peripheral shape as the original insoles. Perhaps that which by now seems to me intuitive, really isn't.
And I still dream of making my own skates, partly for fun, and maybe for others too. A removable moldable and washable liner, made of skin-safe materials, such as certain medical grade materials, would be a perfect starting point. Some high end roller skates, speed skates, and ski boots are already made with such a liner, and some people claim that the results are wonderful.