I'm faced with the same 'opportunity' on my lift-back (sort of an elongated hatch) body; the part has never existed. I have a sheet of textured aluminum about 3x4ft. I may use that if I can find a way to wire-wrap the edge. I also like the idea of making it in perforated sheet metal.
The other half of your question... potential fuel savings? On a less aerodynamic car you might see 1 or 2 mpg. On a high mpg car, less.
You want more — because there
is more. The Morelli fluid tail:
http://ecomodder.com/forum/showthrea...info-2072.html