This is my third design for a filament clip. I noticed that my previous design (https://www.thingiverse.com/thing:3074445) could let the end of the filament fold under itself. The whole point of a filament clip is to prevent that; to prevent the end of the filament from slipping under some other part of the reel, resulting in a tangle or knot later.
See https://www.thingiverse.com/make:558401 for another way of attaching the clip, where the clip is underneath the filament rather than above.
This design keeps the nice "M" shape of the previous design, and adds "wings" that keep it from rotating. The wings are also large enough to keep the clip from bending off of the print bed while printing.
FilamentClip3mm.stl is for 3 mm (2.85 mm) diameter filament.
FilamentClip1p75mm.stl is for 1.75 mm diameter filament.
Choose a filament type that has a little springiness. Avoid brittle types of filament.
To clip this to your filament, grab the end of the filament, and the loop just ahead of it on the reel. Press the clip onto the two pieces of filament.
I chose the wing length to fit even very narrow reels, but be long enough to keep from folding the end of the filament under the rest of the reel.
I first made very narrow, long wings, but quickly found that they had so little surface area that - even in PLA - they tended to peel up from the print bed while printing.
So I made the wings wide enough to stick well to the bed.
FilamentClip3mm.FCStd is the FreeCAD design file for the 3 mm clip.
FilamentClip1p75mm.FCStd is the design file for the 1.75 mm clip (which is just the 3 mm design file with one thickness parameter changed).