Use 8-10mm m4 nuts and screws to secure the quarter-turn mount to the mount arm. Use 15mm m4 nuts and screws to secure the mount to the handlebars.
There are at least a couple other similar quarter-turn mounts out there, but one author says that his broke, and the other only published stl files, and the parts didn't fit together. Plus, I don't like using zip-ties to secure things to my handlebars.
So, I designed my own and published all of the source files, so you can modify it to your heat's content.
I've been using prototypes of this mount for a hundred miles or so, and so far, so good.