In almost every thread concerning the best foam, it's usually mentioned that it eventually needs to be replaced for one reason or another. I tried the camping foam, and it did have permanent (slight) indentations with each use.
And it transfered color if the kydex was good and hot.
Then I tried some carpet pad (got it from a local carpet installer). I think that the high grade pad is fire resistant - because I can aim a heatgun directly at it and it seems impervious to it. It keeps it's shape perfectly, and at this point seems that it'll last forever. I wish that I could tell you what kind (or brand) padding it is, but have no clue. I do know that there are big differences in the cost of "cheap" carpet pad and expensive stuff.
I use two layers glued together on top and bottom.....and get nice detail in the kydex.
They throw tons of it away at a busy carpet place, and it's usually found behind the store.
hope this helps,
p. azz