In my project I need to join two bars in their ends.
This join was intended to be an open-close lock, so I have come up with this coin size interlocking object. You fit the two sides and turn it to lock.
This might also be useful in tight places, or snapping thing to the wall.
(I have also added a "blank" piece, that has nothing to do with the interlock, but has the same design, so it can be used as an end-cup.)
I have attached the SCAD file, so you might want to modify the model according to your needs.
I have print it from PLA, and only a very little adjustment (sanding) was needed to be done.