Grommets! A few weeks ago(at the time of writing this) a grommet separated and came off my keychain. I searched far and wide for an STL because I don't want to buy a pack of 30 grommets and tools to set them just to fix this one old keychain. 

As my search results were dry and lacking, I designed one that would work for many uses and could be resized fairly easy. Even if this doesn't work for you, it should give you a pretty good idea of how to create one yourself.

To secure them together, put the base in the hole you're using the grommet on, dab a little superglue around the exterior “tube” of the cap piece, then press them together quickly.

Specs:
Base:
L: 12mm
W: 12mm
H: 3.70mm

Cap:
L: 12mm
W: 12mm
H: 6.43mm

Combined:
L: 12mm
W: 12mm
H: 6.43mm