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
The full moon is about 31 arc minutes in apparent size. Andromeda is about 190 arc minutes in apparent size. Based on my Eclipse photos at 700mm, the biggest issue you're likely to have with the 300 f/2.8 is picking what part of Andromeda you want to fit in your photo.
I'll have to learn what arc minutes are and sounds fun to try new things