I'm getting a gopro hero 4 silver soon, and I want to be able to mount it on the underbarrel of a shotgun right side up and well (no picatinny on shotguns). The gopro page has a sportsman mount but it is quite expensive (looking at it could provide good basepoint for making one though). If anyone can point me to a mount on here that would be able to do that well or CAD one up for me, I would greatly appreciate it. Also, I would prefer the mount to hold the gopro in it's standard waterproof case (mostly so it will be fine if it rains). Thanks in advance.
Here is a good clamp to start with.
Meant for photography but will hold strong, frankly probably overkill for this application. I personally would put something between the clamp and my shotgun to keep it from scratching. From this you should be able to use existing pieces of photography related designs and GoPro design on Thingiverse to mesh up your own version of the Sportsman mount in Tinkercad quite easily. Good luck!