Bobbin Keeper
These keep a bobbin paired with the spool it was wound from and stop it from unspooling, with tabs to make inserting and removing the bobbin easier. I've tested them with standard Janome, Brother SA156, Class 15, and Class 66 bobbins, and with Mettler, Gutermann, and Coats & Clark spools and they should work with most others too. The leg length is also sized to fit my Wall Control spool holder design. It's on Printables if you want to make your own.