Fits 16mm x 4mm rip fences.
Requires something about the same size as a 6-32 thumb screw and matching nut, the thumb screw I used came from computer case parts and the nut I had lying around.
I printed in PETG without support, just needed to rotate it by -90 deg around X axis. This resulted in a bit of ugliness around the side fins that the GoPro mount snaps into, but otherwise worked flawlessly.
Video from GoPro: https://www.youtube.com/watch?v=4rHlAPJ98to