On-filament clip

This clip tries to address three of the most common issues with most the clips you can find around:

1. While you use a filament, you take off the clip from its spool... and you misplace it;
2. The clip is too loose for some spools, and falls off while in storage or moving them;
3. The clip is too tight for some other spools, and it's hard to put on.

With this clip:

1. You can leave it on the filament while printing;
2. The clip has a very tight opening and a strong spring design in order to clip well on spools with thinner walls (some plastic spools are 2 mm thick on the edge, but 1 mm inside);
3. It has a wide entrance, to make it easy to put it on thicker spools (cardboard spools usually are ~3.5 mm thick).

The hole in this design is for 1.75 mm filament: the hole itself is 2.5 mm, so that the filament can slide freely in it.

Print with 100% infill to have the highest strength in the spring. It works well in PLA, even better in PETG.