I found this design and it fitted my need perfectly. It is a mount for a very popular camera lens used in astrophotography. It not only functions as a weight relief for the camera but also allows to rotate the field of view. Moreover it is possible to focus the lens with the installation of a stepper motor, which will be the next step.
The single parts are screwed together with the help of threaded heat inserts. Just to make sure they are also glued together with epoxy, as the mount really needs to hold up. The screws are locked with Loctite.
Edit: Link to the mount (not my design): https://www.thingiverse.com/thing:6099113
I can recommend this online telescope simulator where you can simulate the field of view with your equipment. Your 300mm lens should be absolutely perfect
Thank you very much