This part is inspired by this thing: https://www.thingiverse.com/thing:1938359 , which unfortunately did not work for me. It was created such that when I loaded it into my slicer, it was too small and my attempts at scaling misfired.
So I decided to make a new one. Furthermore, I wanted a more robust design so that I could apply more force. My part was quite stuck and very difficult to remove. I ended up having to completely remove the drum so that I could put the transmission in a vice and use a large steel wrench to break it free. There is no way that a 3D printed wrench would work, so I didn't even try to make one.
I am including a short tool that can be printed out as a sizing tester before printing out the larger one.
This was created with OpenSCAD. Unfortunately I forgot to save before going to bed, and we had a power outage overnight and I lost the design. :-(
I recommend 3 or 4 shells.