Normally when using a sewing machine, a person has a thread spool and a bobbin (the little spool) loaded with the same kind of thread.  They are separate things, but it makes sense to store them together since they are matched.

This object allows one to store a bobbin with a thread spool. Encountered these in the wild, but figured I could design and make my own.

They are very efficient to print, and a good way to use up filament scraps!