OK So just to confirm it wasn't my imagination running away with me, I measured the stem /handlebar interface area on a Giant and confirmed it to be 31.8 before installing the clamp there. It required the removal of all the rubber spacers, and was just a 30 sec operation using just a Phillips screwdriver.

Lets be clear that the rest of the handlebar isn't that same diameter - it eventually tapers down to 22.2mm where the controls are mounted, and the light can so be mounted anywhere there by adding rubber spacers.

The light itself clips onto the mount with a quick release so its easily removed. These sell here for between $19 and $25 and have a lifetime warranty. So far (2 years) no issues. I use rechargable batteries.