Something to keep the filament from falling off a full reel. For some reason the filament on the reels I have was twisted when it was put on the reel, so every so often it would fall off the side of the reel and it prevented the filament feeding properly. With these clips it doesn't do that anymore.

The scad file is parameterized so you can change it to fit your reels. It's currently set to make 6 of them 45mm long and to fit a 1.9 thick edge on the reel.