it's an interesting idea worth looking into, however what's the advantave over a clear plastic camback? that would be less involving to build less intrusive (could clip onto the trunk etc)
that said, i think the vanes at the sides are going to be critical, the c pillars can often generate vortexes, and while modern cars have much more aerodynamicly shaped pillars you could undo this by placing the touter vanes at the wrong angle (both in top and in rear view)
also it could be interesting to place air intakes like a naca duct there so you'd acually pull the air insight tightening the wake
the resulting profile reminds me a bit of the A2 wich was very low drag
