Dropping the car, rear diffuser will work quite a lot. It NOT BEING A CONVERTABLE. Why do you think the S2ks is so crappy. :]
The Sol has hips. Ideally for volume you'd want a tear drop shape, with the rear end being the skinny part of the drop, its counter intuitive, you'd think the thin part is the part you'd want facing the wind, not so. The water forms into the shape of a tear drop and automatically forms into the best aerodynamic shape due to the pressure and flows, and the fact that suspended water has a flexible container due to surface tension.