So, have you looked at the mount itself, yet? it may have a Screw you remove to find a way to shorten the mounting band underneath.

Or some other way to make the band adapt to the several diameters that handlebars are produced in, these days.

it appears they have a way to do that with the mount band with a few included parts. shorter and longer lower bands and rubber pads..

http://www.topeak.com/products/Lights/tms_sp87