Might be slightly over-engineered, but I always keep a few different sizes on my key-chain. That way I'm always ready, when a zipper pull on or around me breaks. Saved me and my friends a bunch of times already.

Print recommendations: 100% infill; vertical shell perimeter until the loop is fully covered for highest strength; due to very small margin between the “two parts”: avoid over extrusion; no ironing and consider lowering print speed or nozzle temp

CAUTION: Risks of cuts - especially with version 1. If necessary, use pliers to “break” the two rows of teeth gently apart if over-extrusion fused them together.

Installation: Remove any remains of the previous zipper pull. Simply bend the teeth apart to slide the thin outer part through the hole in the slide of your zipper. A bit of force might be necessary - depending on the slide. Once in place snap the teeth back together. Pulling on it only makes the handle hold better together.

EDIT: Version 2 with rounded edges on the inside replaces the original file. That should reduce the need for pliers to detach and reduce the risk of cuts from the sharp edges

EDIT2: I added a version with rounded loop, for those of you who requested it. You can easily scale it to fit your need, but I also added my 3 most used sizes.

Thanks for the feedback so far. Let me know what you think.