A roll of tape and 20 min, and your done! Patience is the key.
The body shop that painted my 300 had a jig mounted to the wall of the paint booth. The 'door' folded down, you inserted the cap, closed the door and latched it with a pin. It masked the cap perfectly for painting. I suppose it was a specialty item for shops that deal with Mercedes body work... probably very expensive, too. Then again, maybe they made it?!
Anyway, mask it yourself and save the labor.