Check out this thread for a sedan boattail (or wingtail). The one pictured eventually blew off the car so use that as a learning experience. It gives 10 to 20% improvement in mpg.
http://ecomodder.com/forum/showthrea...-a-7489-4.html
When I finish my current boat project, I have another wingtail design in mind. With the trailer hitch receiver as the mounting point, use a "tee" welded up from 1-1/4" square tube and from that frame build up a thin plywood boattail using stitch and glue boatbuilding techniques.