I did my engineering degree dissertation on aerodynamics design, specifically with design to the diffuser area of a race car. Without an undertray on the vehicle, the diffuser at the rear will do little more than change the aesthetics. The diffuser is part of the whole system starting under the car and leading out to the open air at the back. A properly designed diffuser actually starts around the middle of the car, and its as it begins to diverge downforce is created at this point. If you were able to flatten out the underbody of the car and lead it into the diffuser at the rear, it will create some downforce, but a curved piece of plastic won't...