I've never had issues with the plastic getting too hot or melting, even with the exhaust running right by it and a catalytic converter 2 inches away from it. It's going to be on the outside and shielded from direct heat by the steel or aluminum that forms the structure of the skidplate. If heat is an issue you probably have other issues like your truck is on fire.
Also, don't know why I forgot to mention this earlier. I've run both HDPE and UHMW in this application. UHMW does seem to wear better than the HDPE, suggest going with UHMW if available but either will work.
I've run this same skidplate set-up on this Jeep for 17 years. I regularly run the hardest trails around here and Oklahoma, Utah, and Colorado in this thing. I've only changed the plastic out one time just because it was looking ragged, it was still totally functional. I had a local shop (Tribe16) put the plastic on this time because they make it look a lot prettier than I could ever get it.
I've stood this Jeep up on the back tires and landed with the full weight of the Jeep on it directly on large boulders multiple times---it's still holding strong and is only slightly bent. This one is built off a crossmember that is .128 wall DOM with 1/4" steel plate and 1/2" HDPE or UHMW. These trucks aren't really built for these kinds of rocks but I hit those kind of trails enough in the Raptor that I want it to slide over rocks easily and not sound like I'm ripping the bottom out of the truck. I'd also rather replace $50 worth of plastic than a $1000 skidplate.
View attachment 162347