In seeing the various materials people have used for air dams, spats, using stuff that needed to be reinforced, have you considered sheet ABS:
ABS Plastic Sheet Textured General Purpose Haircell Finish
This stuff is rigid but not brittle, and very impact resistant. With some practice and the right kind of flat solid tip stuck onto a propane torch, you can weld it. I cut and welded an airbox for a snowmobile engine swap one time. And I have no special skills. If I proceed with an air dam and any other spats, I'll use this stuff.
You see car and light truck mud flaps made out of it all the time.
That's just the first source for it I found. I now see Grainger can ship it to their stores, 1/8" thick, 12x48" for $12.64.
http://www.grainger.com/product/Shee...PPD&pbi=1ZBT4#