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: I added a version 2 with rounded edges on the inside. That should reduce the need for pliers to detach and reduce the risk of cuts from the sharp edges. Thanks for the feedback so far. Let me know what you think.

I also added the stl file for easier remixing.