Not long ago I was in need of a clamp-on FD in 28.6. I mistakenly bought a modern Campy Centaur in 32mm, and when I dropped by my LBS in distress, they gave me a couple of shims to tuck under the clamp. Worked like a charm! The shims were intended for Shimano FD clamps, and are like sections of band. When you crank down the FD clamp, it's all quite secure.