I went to the boat and RV dealership here in town. They sell cup holders that mount wherever you want to mount them. They flip down for use and flip back up when you don't need them. I mounted mine on the driver's door. When not in use the whole thing is about 1 inch thick and about 5 inches square. I plan to put one in my MB. They only cost $6.95 each and can be painted to match.

Kevin