The King Cage Top Cap Mount is a great way to get an extra water bottle cage on your bike. It also lets you fit a full framebag on your bike and still have an easy-to-access bottle cage.
It mounts to the top cap of your stem and provides two drilled holes to mount a water bottle cage (not included).
How effectively the top cap mount holds onto your water bottle depends on both the bottle and the cage. I highly recommend the Cat-Eye BC-100 bottle cage combined with the 0.5 liter SKS cycling bottle – but I would guess that many water bottles will work with that cage.
Top Cap Mounts are made by King Cage in Durango, CO.