Aerohead, this is a question for you :
How would this Kach22i idea work out ?
This is actually exactly what I had in mind by the fences.
Normally the air starts to spill over the sides and meets at the back of the car in one huge vortex. I would think that fences like the one illustrated by Kach22i would minimize the size of the vortex since the main air spilling over the cars roof would not mix with the spiraling air on either side of the car.