If you have a 3D printer and don't want to spend on a mount for a GoPro action camera, why not just print out one? That's what iambatsloth exactly did... "I've been super into airsoft lately and want to document my experience. The best way to do that is with a GoPro but I don't want to spend any more money on such an expensive hobby. So I 3D printed a 20mm Picatinny GoPro mount. Let's see how it holds up.

*Massive thanks to Alex for letting me use his gear."