Had an idea that relates to another thread I just started about using window heat shrink plastic to mock up aero body mods. You could use the same thing to make a smooth wheel cover to test any gains, plus it would be clear so you could see your rims. Might not hold up well over time though, but it should be cheap and easy. You could even use it as a mold to make a fiberglass version, unless the polyester resin dissolves it
You'd have to do some testing.