[Phantom]

I would get few high CFM computer fans and mount them in the trunk on the exhaust flap (not sure that is what it is called). You should see what I'm talking about if you remove the carpet by the rear light it is a little box with a rubber flap that you can push open.

By powering the fans and pushing air out of that location it will draw air from inside the car keeping it cooler especially if you leave the HVAC vents open (not set to recirculate). Some one did this on cleanmpg a few years ago.