If you are are used to hacking GoPro cameras (e.g. you make custom rigs for VR) you may have noticed that your custom lenses get out of focus pretty easily. Once, in an important shot I turned slightly the lens while taking off its cap and almost got killed by the production team. So I made this small accessory that helps fixing the lens. It uses a 3mm nut and bolt. The bolt is inside the holder. Depending on your printer settings you may need to push it a bit with a soldering iron. Also, you may want to sand it a little bit in order to make it fit.
Get this from thingiverse or buy it from shapeways.