[lunarhighway]

i really like this thread and i can't wait to see the final result but if it was me constructing this i'd look into sheet aluminum
(used it once for my undertray an now i'm in love with the stuff, such a pleasure to work with...)
... since you've already build a working version out of carboard i"d just transfer the shapes to some very thin aluminum carefully bring it to shape and rivet (or maybe glue) it together.. if you can find a sheep big enough you could even make this a one piece affair. add perhaps a curved metal strip at the back for strength.

it would still be the lightest thing on the car, be relatively strong , and if you cared about looks it could be painted to match the rest of the car, although natural aluminum finish would be cool too. since this is how most WW2 era planes where constructed it's not the worst option either

anyway, i hope the foam thing works out, as i'd really love to see this thing come together