This is quite custom mount ideally suitable for mounting GoPro Session behind the scope, using Picatinny rail.
It was designed to sit on Hatsan Gladius airgun, with Leapers UTG Compact 3-12x44 30 mm AO MilDot EZ-Tap scope. Still, it may be relevant for other setups as well.

There is a thin plate included in the bottom of the design that links the main body with the GoPro link section, to prevent small elements from moving around on the table in beginning. This should be cut off after printing.