Filament Spool Keeper

thingiverse

This is a standard 30mm OD retainer that slips smoothly into place. A spool with an ID of over 50mm can be easily installed without any adjustments - simply slide it onto the retainer. However, for IDs between 30mm and 50mm, you'll need to loosen the 30mm retainer by about 7mm before removing the keeper. Once the keeper is out of the way, you can slip on the spool and then replace the keeper. The process might sound a bit involved, but it's actually quite straightforward.