EDIT/WARNING:

Even in PETG, the original ("Long") mount failed on the first big bump in the road.

I have redesigned this to a 'stubby' version which still allows for some rotation of the light beam (and clearance of the shifter cables), but it's tight. This was done to reduce the moment arm of the initial version and hopefully avoid failure. I HAVE NOT TESTED THIS YET. I've also changed print settings (higher end temp and NO layer cooling) to try for better adhesion between layers.

Use with caution, definitely tether your light, and/or look for another solution.

I've left the original design in case someone want to try other materials, but I'm going with the 'stubby' version.

This is a Cygolite Metro (and others) mount for Trek/Bontrager Blendr system (similar/same as GoPro?). Remixed from the clip and slide glovesforhands made, I added the arm with the mount point. Printer calibration has to be pretty spot on for this to fit.

You need a small bolt/nut to attach the clip to make it replaceable if it breaks. No idea on size (will measure later), I had some sitting around in my spares bins. The nut fit perfectly in the recess in the clip.

A few notes on printing:
I printed this with the narrow end face-down with tree support (Cura Slicer) holding up the 'ears.' Printing in a more flat orientation, the rails where the light mounts were more likely to separate.

Also, I printed the clip with the width scaled to 95%, otherwise the fit was rather snug, even with printer calibration. Do NOT use 'uniform scaling' - leave length and height 100% or the catch won't line up correctly.

Initial runs in PLA were too brittle (aforementioned rails as well as 'ears' breaking off around the bolt holes), but that could have been down to my filament condition. I finally printed it in PETG, and this feels more robust, and there's a little more flex in the clip/release. To be safe, I added a tether to my Cygolite Metro anyway.

Only after producing this, I found that Cygolite actually has a 'GoPro' compatible mount which would probably work with a small spacer arm, but here we are.

Special thanks to toyotaboy02 for cleaning up my model a bit for me.