I couldn't find a ring hook that was reliable enough to mount my pull-up exercise balls on Thingiverse, so I redesigned one from scratch.
This ring is detachable and it has a safety lock (the top cover) which also provides extra strenght by keeping the two halves of the base together.
I can hang my full body weight from these. I have a box full of failed designs before I figured out how to make them strong enough though. I think it's cool that it's possible to print a functional part that works for a purpose that would normally use steel.