SKS Anywhere Bottle Cage Adapter Including Topcage:

The ANYWHERE bottle cage adapter really lives up to its name. You can mount it anywhere on the bike frame, even where there are no mounting holes. Available with or without the TOPCAGE bottle cage, its rubberised straps provide a firm and stable fit to tubes of any shape in diameters between 50 and 80mm. The velcro fastening means it can be removed in seconds.