Been wanting to put a flat underbody on my MR2 for a while now, beyond just improving MPG (front end gets floaty at high speeds due to the low weight). I was looking at aluminum, but it gets pretty expensive, and can dent easily. A bit flimsy, too, if you want to keep weight down. Then I checked out plastic, which is much more durable, but would need to be thick to keep it rigid, which means even more expensive and heavy.
Finally, like most people here, I decided on coroplast, which is cheap, lightweight, and durable. The only negative is that the average campaign sign is pretty flimsy overall, and would need significant bracing behind it to maintain its shape.
Searched and searched, and I finally came upon the ultimate cororplast: polycarbonate!
http://www.mcmaster.com/catalog/115/.../84825kp1l.gif
I found it at McMaster.com, listed as noncorrugated plastic sheets (interesting, since coroplast = corrugated plastic ;) ). It's actually made for indoor/outdoor wall and roof construction. I bought a 12x24 test sheet, and it just came in yesterday. This stuff is extremely rigid compared to standard coroplast, yet just as durable (strong, but not rigid enough to crack). McMaster even says it's virtually indestructable! It's UV, weather, and chemical resistant, so it's ideal for outdoor environments and leaky cars. :p It's also clear, making finding mounting points a breeze once it's cut to shape and fitted in place. It's heat resistant to 250F, so you'd only need to put heat shielding around the exhaust area. This stuff might even work well as a grill block.
For anyone who's interested, it at McMaster.com's online catalog on page 2080, listed under "Indoor/Outdoor Polycarbonate Wall and Roof Panels". Of course, this would be more expensive than snagging some campaign signs when nobody's looking, but you can create an extremely professional looking setup with it that won't flap in the breeze.
