The radius of the mount is 1"
I wrap gaff tape around my handlebars to make it fit without slipping.
For the button hole in the top, make sure you print it with the flat side towards the bed so the printer doesn't have to bridge that gap (like in the main image). You'll need support material, but if you're settings are good, it'll come out without a problem. In Cura, I set me support airgap to .2mm and the x/y offset to .7mm
This will save you the $30 of buying a mount from GoPro :)