Is there a plan to fabricate Plex or Lexan covers to cover the void behind the taillamps?
This area is critical with a fastback body. It needs the tumblehome and curvature to mitigate formation of vortices, blending the 'sides' into the 'roof.'
If you can't do it, you might as well foam the whole area in, on both sides, and put a trailer light kit on the back of the tail.
According to Jeff Powel and associates in England, you could be introducing, something on the order of a 33% drag liability.
