Most of my filament spools fall into two categories: cardboard spools with holes around the rim and plastic spools with grooves around the rim.

These clips were designed to handle those two types. The “cardboard” one (red in the cover photo) has a bump that fits into a hole on the rim of a cardboard spool. The “plastic” one has a bump that fits into the groove on the rim of a plastic spool.

These two designs work with nearly all of the spools I have, except for the Prusament spools. I haven't designed a clip for those yet.

Update: There's now a clip for Prusa spools.

I also altered the contours of the clip so that they sit flat against the outside of the spool, making them less likely to catch on something and come loose.

Printing is straightforward and they're a great way to use up leftover filament.

I keep a bunch of them handy in a small container, color-coded so I can easily identify them.