I have that Twofish velcro cage, and find that I can't get it as secure as I'd like. Mounted on the top tube or the front of the seatpost, there's enough give that the bottle both rotates around the tube and twists side-to-side (i.e., doesn't stay parallel to the tube), such that it gets in the way of my legs. Mounted behind the seatpost works okay, though, and that's been my go-to solution so far for carrying two bottles (along with a cage mounted to the holes in the steering riser).
I have one of these on order from Amazon:
http://www.minourausa.com/english/ac.../bh100m-e.html. It's a quick-release clamp for 28-35mm tubes.