The yellow piece that warped in the front was a a piece of 'foam board' (but stiffer and thicker than retail) It was on clearance and was yellow.
In retrospect, coroplast is really the best product.
Since you have seen the pics, let me answer some thoughts.
I have never had a heat problem with my current setup which has now been in place a couple of years.
I have had minor 'adhersion' or mounting problems which I have ackowledged. The moving air is pretty forceful and can peel up coners and edged. It can also 'move' coroplast if not anchored well.
I had my front piece completely blown out by the air coming thru that lower opening. I hadnt achored the back edge well enough and it blew it out befor i got past 40mph
I have found that there is plenty of air moving between the engine parts and the coroplast. As long as your not touching your good. I try to keep the coroplast about 1 inch away from exhaust parts and have never had a problem.
Sreening helps disapate the heat.
Best of luck.
PM if you want to ask any specific questions.