This is a GoPro mount designed to float. The part was designed to accept 3 standard bottles. Without an o-ring, the threads are not air-tight but the seal is sufficient to keep water out when floating on the water. I also added in a small hole where one could tie a string or a fishing line.
Print it with the triangle base on the printer bed, not as it is in the STL file. While printing with supports touching the build plate was not necessary on my Monoprice Mini, it may be necessary on your printer. Try it out without supports first and if it doesn't work, add supports touching the build plate.
The knob should be printed with a brim so it doesn't fall over. It uses an M5 x 50mm hex bolt and an M5 nut to secure it.