It was, in fact, that car that inspired me to think in that direction.
I have a hunk of white thermoplastic that may be OK to make a prototype. I think that if I do a permanent one it will be out of acrylic or polycarbonate. (Those are both thermoplastic, aren't they?) A smoke colored one could have some good effects visually as well as aerodynamically.
My goal is a reliable 50+mpg on the highway, which should be attainable.