GoPro Plastic Bottle Cap Mount
The idea is to use any plastic bottle as a grip/stand/floaty at the same time.
I saw plenty of solutions here on Thingiverse as well, but I did not find any of them perfect.
The concept is brilliant, you get basically a free GoPro grip, which at the same time makes your camera float. Though a 3D print is most likely not 100% watertight, it is good enough to make your GoPro float in water.

I designed it 3D-print friendly, but also with common engineering sense:

• no need for supports, the big inner overhang is sloped
• sharp edges are rounded: especially important in high stress areas
• also the edges sitting on the printbed are smoothened: they could cut your hand otherwise when tightening the cap
• walls are thick enough that some infill will be printed to get a sturdy part (two other prints I tried from the site where bendy, weak)
• yet tried to keep size to the minimum not to get a bulky part
• surface is grooved to create better grip

I used the standard thread called PCO1881. This is the newer style - lightweight and lower profile, against the older PCO1810. Outside diameter is 27.4 mm and pitch is 2.7 mm.

The drawing was printed, used and tested diving in a lake with the GoPro Session and I was satisfied with it.
I hope you will enjoy it!