I've run across this problem before on an 'EL-300' light. You need some kind of soft rubber material, like rubber splicing tape, which was what I used. You could also try a section of old cork tape, or anything grippy to act as a spacer between the bars and the inside of the mount.

Rubber splicing tape can be found at electrical supply houses, or look around at electrical departments in Wal-mart, Lowes, Home Depot.