Really all you need is a piece of foam and something to cut it with. I didn’t get really scientific here. I grabbed some of the foam that comes in fishing reel boxes to protect the reel and a pair of good scissors and started cutting up small rectangle inserts from the foam piece. Now I will admit there probably is some sort of foam that might react with the plastics and warp or otherwise mess up in the heat.
I’m not a chemical engineer, and I didn’t test different types of foam. So you might have to do a little experimenting there. But the foam I used did work in my testing of this idea.