I have such a mounting on one of my bikes. I bought a Blackburn mounting bracket from Performance for this (about $5) since the Topeak mount wanted to take up the water bottle bracket. It sits nicely just off the side of the cage.

Now, on my vacation bike, because it was a custom built steel frame bike, I had water bottle mounts placed underneath the top tube so I could mount my Road Morph ala' the older style full-length frame pumps. Very convenient, but I suppose a custom frame is a bit of an expensive pump mounting bracket