The best shape is definately going to be a tear drop. Per dollar spent, I think you would see the biggest effect.
Phil Knox's Truck
Simple, Quality:
1/2" Foam Board,
Fiberglass,
Epoxy Resin
composite construction, faceted sides
Simple, Cheap:
Lightweight Tubular Skeleton
Coroplast skin
Duct Tape
faceted sides
If you just want a normal tonneau, you could buy a bunch of aluminum siding and pop-rivet yourself one by forming ribs w/ flanges attached to skins. Cheap, easy, and unique...
- LostCause