A few years ago, the zipper box broke off my son's new winter coat.  I poked around the local craft shop for a replacement, but didn't like the looks of any of the offerings.  The zipper itself is plastic, so I figured why not print a fix?  I threw together a design in OpenSCAD, and after a few iterations, it seemed to fit pretty well.  The design is a basic block with slots for the zipper backing and for the other half (tongue?).

There's not much to the design.  I could have parametrized things for easier changes, but considered this a one-off project.

// ykk zipper stop
epsilon = 0.1;
difference() {
    translate([-6, -6/2, 0]) {
      cube([10, 6, 10]);
    translate([-7/2, -2.8/2, 0]) {
      cube([2.7, 2.8, 12]);
  translate([-3, -2.0/2, 3]) {
    cube([2.0, 2.0, 12+2*epsilon]);
  translate([0, -3.2/2, 3]) {
    cube([3, 3.2, 14-3+epsilon]);
  translate([-7, -1/2, 3]) {
    cube([5, 1, 14+2*epsilon]);
  translate([1, -1/2, 3]) {
    cube([4, 1, 10+2*epsilon]);

I printed the final part in red ABS at a 100-micron layer height, and used Duco Cement (awesome for ABS) to attach the stop to the zipper:

He wore that jacket for two seasons until he grew out of it, and the zipper is still going strong.

I fixed another jacket (same brand, but with the zipper on the other side) one year after this (in silver ABS), but I think it got donated already, so I can't show any pictures.