For anyone considering making grill blocks wheel well skirts or other mods that will be in constant view on the exterior of the car ...
I learned of this
Allstar ALL22400 Black Plastic Roll from Xist. (0.07" Thick 24" Wide 10'). Xist used it for an airdam two years ago. (Thanks Xist!)
Pros: It is better looking than coroplast by far, does not fade in the sun, and has flexibility and rigidity characteristics that are pretty cool.
Cons: It flexes just enough to change shape slightly when it gets hot, either from sun or engine heat. It is also a little hard to cut cleanly and regularly. You'll want to experiment but I have gotten some really good results with razor cutting tools for lexan. I prefer the look and durability of this stuff to coroplast by a mile.
Here is what my roll looked like the day it arrived: