I was looking for a simple mount to place my Hero 6 on a post for my daughters sand volleyball tournaments. I came up with a simple mount that uses sleeping bag straps to mount to any post. The actual straps I used can be found on Amazon but any strap that's around 2cm wide will work.

https://www.amazon.com/gp/product/B000FAPJLK/ref=oh_aui_detailpage_o01_s01?ie=UTF8&psc=1

V1 is is the strongest but prevents the camera from tilting parallel to post. If you have a Hero 6 black and use the skeleton frame this mount works great. If you have a larger waterproof case the angle will be greater and the V2 might be better.

V2 is notched to allow the camera to be parallel to the post. Note the mounting arms are weaker due to the notch.