My Topeak beam rack (post #6 above) used a rubber shim, I can't recall if that was to make the seatpost match the clamp diameter, or the clamp was V-block style and the rubber gave good grip without scratching the seatpost. I think my clamp was round and they later went to V. Been a couple decades since seeing it. It stayed in place fine, plus I think it looked good, and kept the bags aero-sheltered behind my torso. It had a very small amount of forward tilt, due to the steep seat tube angle on that racer, I think 74 degrees.