When I made a full belly pan for Turtle, I used an aluminum sheet under the cat and pallet liner under the exhaust. It's been on the car for way over a year with no ill-effects. Actually, the whole belly pan looks about the same as it did when I installed it.
I can't stress enough how awesome the pallet liners are to work with. They are way stronger than the coroplast you get at Home Depot and they haven't shown any signs of wear from road debris (and they're two bucks each!).
http://ecomodder.com/forum/showthrea...tml#post507791