In-spool 1.75mm filament clip.
I tested 10+ clips and didn't like any of them. Received something similar on a spool of Solutech filament I purchased, so I thought I would design my own similar clip (after the fact, I also found another similar clip here on Thingiverse - posted link as remix even thought it truly is not). Therefore, I do not claim this is an original design even though it is proving to be quite popular.
I like that it sets inside the spool and does not clip to the edge. Simply unrap one layer to attach the clip, wrap filement around again and attach. When in position, push filament down to secure. These are the absolute easiest on/off clips I've used.
Simple, but elegant solution. The only think you have to remember is to uninstall it when mounting the spool (I've forgotten a couple of times, but clip managed to pop off on its own during filament feed).