As merv has pointed out, the best stuff is aluminiumised cloth material. We used that on our Harrison Cobra for engine bay. I've seen it used on the underneath side of rear clips on GT40s. On GT40s it's been edged.
High-temp silicone works well to attach it.
More information and photos here:
http://www.snakebite.com.au/firewall...tre-p-361.html