There are two ways of looking at this:
1) Use the technique to build a cover and use it only as a cover.
2) Build a boat that can substitute for a sort of cover - but it is primarily a boat, perhaps modified a bit to fit the shape of the back of the cab. For example, look at this design, pointing the nose backwards of course:
Black Fly 8 - Geodesic AiroLITE Boats
You might eliminate the curve (sheer?) in the gunwales so they sit flat on the edge of the pickup bed. The nose of the boat would overhang the back of the truck a couple feet.