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Check out instructables.com and use the search for a DIY vacuum forming setup (using the oven and a vacuum cleaner.)

You can use that to make the plugs for the backs of your wheels, in any color you want, including clear (not a good choice, for the brake dust.) The plugs aren't "THE ANSWER", so to speak, but they prevent cross flow through the wheels, which is fairly turbulent. If you use the plugs, be careful about long periods of heavy braking, or several sudden brake lunges... they may overheat.