I was cleaning up the garage and came across this belt that I use to carry a water bottle with me when I run. It was black, looked kinda cool, and held a water bottle firmly in place. Then I turned and looked at the tank bag on my RT...Hmmm???? Sure enough, it fits so slick around the tank bag that I think it actually looks like a BMW accessory!
These belts are available at any running/athletic store. Some hold multiple bottles.