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
2006 Inferno Red Ram 3500 MegaCab 3500
2010 Rescue Green 2 door Wrangler Mountain Edition