One of the ends of my backpack's zipper broke, allowing the zipper to zip all the way off! This little trinket clips on to the zipper tracks, preventing the zipper from unzipping too far!
I made both a version that clips on, and one that has some little slots to allow stitching it in place.
Really, though, the clip is good enough in my case.
Put that zipper back onto its track things!
Then clip the ends with this, so it stays on the right track.
Then get tempted to sew an extruder gear onto the bag while you're at it.
Oh, and don't forget to print it out =)