THERE CAN BE ONLY ONE!
I run both 1.75 and 2.85 filaments in my printers and grew tired of having to dig around to find the right size when switching filaments. Basically I was aiming for a clip that would secure to any manufacturers spool in any filament size within the common deviation ranges of 1.75 and 2.85. So far this has worked great! They do not protrude outside of the spool dimensions by much at all so spools still stack and pack together well in dry boxes.
If it isnt obvious, how it works is that the outer most legs deliver the actual clamping pressure and squeeze the third leg... against the inside of the spool. The two longer legs have bulbs to allow adequate clamping even on certain spools that have a thicker outer rim and then recess in for saving plastic in manufacturing. I have not had a spool yet that this didnt work on and you can get the filament surprisingly tight when you slide the clip around the rim until there is a step of any kind that impedes sliding.
The protrusion(faces inside the spool) is for your thumb to make opening up the jaws easier while inserting filament.
Though there are some very thin parts of this I have been able to print them using a .5mm nozzle with very minor post processing. I just printed sets of three of them a few times with different settings until I was happy and started printing full beds of them using those settings. Totally worth it!
I used concentric exterior infill and made every layer an exterior layer to provide maximum strength in the direction of applied force.