would be to use some black plastic or a sheet of alum that would meet flush with the bottom lip of the rear bumper and then angle up to touch the underbody of the car, basicaly diverting the air downward instead of into the bumper. But i am afraid this would cause some amount of lift. I guess thats why most of the properly designed diffusers extend past the bumper and curve upward. I just dont want some gaudy peice of alum sticking out from my bumper