[lunarhighway]

i've used very thin alluminum for my undertray and the trick is to fold up the defes to form a some sort of ream tray also strenthening the corners will dramatically increase the strenght. some aditional attachement points will make it all rigid.

mine still rattles a little bit when i slam a door, but on the road it holds up beautifully.
if cost and wheight are a gonsidderation go as thin as you can.