I was looking at some of the tonneau cover threads here and noticed nobody here has used Platt Monfort's ideas for construction of such covers, or any other aero shape. Take a look at his website:
Geodesic AiroLITE Boats - ultra lightweight SOF canoes and boats; plans, projects and tutorials
He's no longer with us but his wife is selling his designs now. I am in the process of building one of them, the Whitehall Jr., and it just struck me how well this technology could be used in an ecomodder project. He uses dacron shrink-wrapped over ribs.
There are various versions and copies. You can see some on youtube, for example:
The neat thing is that after you have your tonneau cover done like this, you can take if off your truck and use it as a boat! Well, maybe not a very pretty boat, but it's at least worth thinking about. Anyway this might be a way to get a very light shape that can be curved in two directions...