Ive thought about this many many times. However, my thoughts are to maximize the function, and so other things would need to be altered, which are currently in the way, like the engine and body. You could take some of the suggestions and maybe it would help in some way. I havent tried any of this though.
The rear diffuser basically is air flow management for air moving under the car. With the current shape of the car that isnt happening in any kind of laminar way what so ever, so a diffuser at the end of the run would be kind of pointless, as it wouldnt be managing much. The perfect shape of the body's underneath would be an "upside down airplane wing". So towards the back of the car, the floor would need to gently sweep UP. The high and low pressures will pull the air out from under the car, promoting "down-force".
Working from the front back;
1) I think you would need a splitter, and a redesigned grill opening to pull air in.
2) You either need an extractor hood, or reworked fenders to reduce the pressure in the engine bay. The Radiator(s) would be baffled off, and a clear path constructed through baffles/ductwork exiting the hood or fenders.
3) The splitter would pass the remaining air underneath the car, which would need to have basically a body pan promoting laminar flow.
4) Towards the back of the car, you want the floor to sweep upwards, to almost the height of the trunk lid if possible. Because this would totally destroy the body, maybe the license plate bracket is a better target. Fuel tank, and spare tire are would need to go.
A diffuser could then be attached at the back, likely where the bumper/license plate brackets are to manage the air being sucked out the back. Lowering the ride height would help.
Anything to help the air be more laminar underneath will help the downforce. Packaging is obviously the issue here.
