I've tinkered through 6 different versions of filament clip but this is the first one I like enough to share. It has a nice wide looping clip for strength and durability but I made the outside part of the clip relatively thin and flat. This means that it can easily fit inside the original carton or, as shown in the pictures, you can even stack spools with the clip in place. I keep mine in stacks of two on a shelf and the clips make barely any gap.
For the filament hole I used a 2.2mm bore hole but I flared it out to 2.8 mm on each end (so it's reversible) to make it easy to press the cut end of your filament into the hole. Once it goes through it's quite grippy, enough so that I can pull the filament tight and it stays in place on the shelf.
I use the one with the clip on the end because it makes it easy to see which filament it is when looking at the spool from the top or bottom. I've also included one where the clip is identical but I've moved the filament hole into the lower loop of the clip. This might be better for spools that are stored standing upright since it will keep the filament within the confines of the spool.
Would be interested in any feedback or suggestions. Happy printing!
Prints well on it's side (i.e. with the filament hole oriented vertically). No support or rafts needed. If it were oriented the other way the clip would likely break at a seam when stretched onto the rim.