My mom bought a flexible phone mount from a second hand store only to realize that the thumb screw that was meant to tighten down on the ball socket had broken and fallen off at some point in its life so the ball and socket joint was impossible to immobilize.
I was able to cut the original ball off (in order to slot a new thumb screw into it), then model and 3d print a replacement ball joint, as well as a thumb screw (matching the threads from the mount) so that the assembly could be put back together and made functional again.
Very happy to see something avoid the landfill over such a simple part being broken, and always happy to fix something for my mom.
Fusion360 file included in case anyone else ever needs to use the design for something similar.
The author marked this model as their own original creation.