I like ABS plastic as it holds shape once you heat and form it the way you want it.
Ridged foam could work but you have to figure out how to keep it in place with all the air pressure against it, I have used it in some places on vehicles, semi ridged worked well also.
The IC will not get hot enough to melt at least most if not all plastics.
The top and bottom plate on the IC is solid aluminum, using a stop sleeve on a drill bit you can drill 1/8" holes and rivet the ABS to the top, bottom if needed, no worries as long as careful to not damage the passage by going to deep. I did this on the J'Line, will likely do so on the DHM.
If worried about that you can likely use the top grade 3M trim tape instead, it handles heat on a windshield holding on mirrors, etc which is probably much hotter.