Getting back to the original focus of the thread, there are several examples of underbody panels linked on "60+ Efficiency Mods".
60+ Vehicle modifications for better fuel economy - EcoModder.com
Here's another coroplast example:
http://ecomodder.com/forum/showthrea...shed-1289.html
And here's the first car MetroMPG was referring to, which used window screen and paint:
http://ecomodder.com/forum/showthrea...tors-1209.html