[MetroMPG]

Another thought: start with a flat, slightly upward angled undertray. Tuft test it. Tufts are your friends. (Use a remote camera to record the results.)

If you find that there's attached flow on your undertray, you can study the direction of the tufts to discover flow direction. If you put a "fin" at any other angle than the direction indicated by the tufts, you'll probably be increasing drag through vortex formation off the fin(s).