I use the blue sleeping mat sold at walmart but I don't think that is what your looking for. I was told once by a maker that something called "blow molding" works great. He described it as being used in the industrial molding process. I never did find any so can't say how well it works.
hey boss , I tell you what works great and is pretty heat resistant . the brown sanding pads about 1/4 in. thick . been using them for my kydex press for a while . I adjust the sheath size by adding or taking away pads . I buy belts and stuff from wwwsupergrits.com and they send a couple of pads each time I order something .http://www.supergrit.com/
I got mine years ago from K&G, it's 1" thick PVC sponge rubber or something like that (they don't seem to carry it anymore) , Knife kits has it but they don't say what it's called.
I have 6 different types of sheet foam now. I will be testing this weekend and figuring out good, better, best in use. In looking at the stuff, I think the premium neoprene is going to give the best impression.
When I made 4 camp knives, I gave them concealex sheaths, and I made a "press" using plywood and the yellow carpet foam. It's pretty firm, but has some give.
It was just from Menards, and it was like the quality foam. It's also waterproof on one side... Just what I used.
Maybe like this foam http://informatorbudownictwa.pl/images/stories/19894:1_76.jpg - he cooled THERMAFLEX FOAM or TUBOLIT and he is using at isolate hot water pipe. When I buy some i will be testing. The foam is similar like foam from knifekits.com seems to me. I suppose that foam is a family of polythene foams about the right consistency.