I cut a piece of narrow inner tube so 18" long, wrap it around the HB, try the mount on it and just keep cutting an inch or two at a time until the fit is good.

I'm not just saying this. All my HBs are 25.4 or 26.0. Nevgr owned a 31.8. I've got an Element Bolt on my good bike right now. (This trick works for all oversized HB mounts. I just keep that length of tube with it's computer, light or what-have-you. For a neater look, you can cut the tube into a narrow strip just the width of the clamp. but that required more than twice the wraps. I used to do that but no longer.