I needed a basic set-screw shaft coupler, but unbelievably, I couldn't find a customizable one on Thingiverse, so I made one.
(Note: if you want a clamp-style shaft coupler, check out this thing by toxnico).
This coupler should be used is with TWO set screws per shaft, each at 90 degree angles to one another.
Four holes are provided for each shaft in case you strip one out, but you should not put more than two screws in per shaft. If you do, the shaft can pivot on the points of the two opposing screws instead of being pressed flush against the side of the coupler like it should be.
If you're concerned about weight asymmetry, use two holes on one side for one shaft and two holes the opposite side for the other shaft.