Since its almost summer I decided to try and make a GoPro mount to film pool parties from a unique angle. This makes the GoPro sit just under the surface of the water. I plan on adding a counterbalance soon so that it can also film above the water. The mount is designed so you shouldn't need a nut for the backside of the thumbscrew.
I recommend printing without supports. Print two of the pieces. Connect the corners with string or rubber bands. Whatever is long enough for the ball you're using. After this insert a slightly deflated ball and then inflate to stretch out the strings. Adjust as needed.