One of my winter projects is getting a small piece of steel pipe the same OD as a 12 oz soda can and welding a pipe with an ID that will hold a 20 oz bottle then powder coating the whole thing black. I have the Leather Z armrest and love that part, but the cup holder will still only hold 12 oz cans.
1999 Boston Green M Coupe
2002 Z3 2.5 Automatic Roadster
2013 Ram 3500 Crew Cab Dually, 850 ft/lbs torque, stock and staying that way
1995 318i spray paint red, 170,000 miles and climbing