This is an oldham coupler intended to interface the X/Y axes of a mini mill to a stepper motor, but the general design principles can be applied to any interface where shafts might not be perfectly aligned. The one innovation i am "claiming" is using the two part motor coupler to create a secure threaded connection to the motor shaft. Hot melt nuts are embedded into the square insert part which is then press fit into the coupler. This allows it to retain screws without pull-through issues. This was sized to fit ~ 12mm M3 screw.

If lacking hot melts, enlarge the slots to fit regular M3 nuts.