Here's a concept I think may have some potential. It's like a small (and I think legal) version of the chimney cooling exits from pre-2009. The advantage of this system is that it keeps the turbulent cooling air away from the bodywork (mostly) and reduces drag over a simple slot. It also allows the sidepods to be much tighter as air is being extracted further forward than with the existing barrel exit at the rear.
I'm not sure about size. I'm sure there are rules limiting the amount of volume the exit can have in certain projections. Also I thought exits in this area were banned but then I remembered this from china 2011:
What do people think? Is it legal?