[elhigh]

A flat sheet won't do compound curves, but you can cut darts and stitch the pieces together to create some. It'll look a bit like the Hindenburg.

Obviously you want less frontal area than more, and the sooner you can start the taper rearward, the better, if only to slam the air closed behind you more gently.

The lowest w-h I've seen for an electric car was right at 100 w-h per mile, and that was a nearly full sized car. I don't remember how fast it was going, but it wasn't like an Electrothon vehicle, it was moving at traffic speed. The best plug-in hybrid in the EM Garage has a PB of 177 w-h/mi, so getting a recumbent trike to do a good speed at just 100w input should be pretty straightforward. That 100w of rider input might be all it takes.