Here goes my current test setup. My current wheel spokes come more outer than rim so I have black soft material to bring that cover littlle bit outwards. Now the polykarbonate should not get scrapes... At the moment it comes too much off there is about 13 mm gap between rim and hubcap.
polykarbonate hubcap.jpg - Windows Live
This way you dont need to drill two holes to your hubcaps, but the attachement is difficult. With this you need to attach it behind/inside the wheel. Better version is coming which will solve that problem and that will be the serial version
. Serial versions attaching time estimation is one minut per capsel, if those holes are already drilled...