Louvers, and lots of them:
This is of interest to me because my platform of choice has a handy source of 1000—1200 cubic feet per minute of engine cooling air pumped through two 4x5" ducts right into the wake. I don't care about the exhaust because it is dirty and corrosive.
I'm wondering whether it would be better to dump into the middle of the wake, or have vents around the periphery. Maybe even a few inches ahead of the rear bulkhead so it adds energy to the air just before it leaves the body. Like the Renault R8:
