[Christ]

If you have stake pockets, you can get some green wood (saplings) and stick them in the stake pockets, then bend/tie them into an arc behind the cab, and a much smaller arc at the end of the bed, and tightly wrap any kind of cloth to them... that would give you a crude way to test an aero cap on your truck.

You can also make the frame with old brake line, if it's fairly clean and you know how to weld. If you want it to seem less fruity (random fabrics - shower curtains lol) you can use thin chicken wire or something, which will act about the same as fabric at speed.

I think Big Dave has a fabric cover on his Ford.