This tiny thing is a must have for pro users storing their spools with material (for example in boxes with dehumidifier). I have tested many filament clips during years but every one has some flaws – from annoying everyday use to unreliable attachment. There is no reason to be frustrated by the loose or entangled material anymore.
These two clips for the most common 1.75 and 2.85mm filaments were designed from scratch to suit spools of Fillamentum brand (old black and new transparent). You can 3D print them very fast and with minimum of material. The same clips could work also with some other spool types, I have tested standard 0.75 to 1kg spools from Innofil3D, Prusa, older Ultimaker's spools etc. Just test yours.
EDIT 27. 10. 2018: Added the variant (V4) of 2.85mm clip with a bit bigger inner diameter (3.1mm instead of 3mm) for filaments with wider tolerances or printers+slicers with different settings.
Tip: Every 3D printer has a bit different tolerances (and some materials too). → If your clip is too loose or tight, just use resize function for several percent more or less in your slicer.
Clips are tested on Prusa i3 MK2 and Ultimaker 3 with 0.2mm layers. If your 3D printer is calibrated different way, just use scale compensation if you find them too loose or tight.