John,
Duke and I may need to add a fin to our car, depending on what Woody comes up with but I am planning to use a piece of 1/4 inch aluminum plate cut to the area we need and having leading edge that lays back about 60+ degrees, I would also champfer the leading edge and make the the exact center of the champfers be a sharp edge. Read about "vortex lift in "Race Car Aerodynamics" by Joseph Katz which has a great section on it. As the car starts to spin side ways the sharp leading edge develops a vortex that runs down the back side of the plate, a vortex is low pressure so it makes a pressure differential on the tail that is pushing in the opposite direction to the spin. The amount of "lifting force" starts small but increase rapidly as the angle gets larger and will work fairly well at angles higher than 30 deg. This appears to me as being just what we need, easy to fabricate and mount, anti spin force rapidly increases as the car gets more side ways and probably not much more drag than an aerofoil shape.
Rex