i dont think there is anything that will do it. rhino's arent sized to fit 20 oz's. Redz, however, does make a good quality neoprene cover specificly for flat bottomed 20 oz's.
redz do a nice one but why spend that money when you can make your own for like a pittance?!?!? buy some neoprene from a wholesalers and make one they aint that hard to do.