MakerFarm Prusa i3v Z Axis Couplers. Dimensioned .022" over to allow for material shrinkage. The stepper side of the coupler fits nicely onto the stepper shaft with minimal interference. Clean up the lead screw side I.D. with a .187" drill. Very little material will be removed. It will thread straight onto the lead screw. Make sure to clean up coupler mating surfaces so both sides fit together to your liking. This should be minimal as well.
Use version 2 if you would like the couplers to snap together easier. Also resized the lead screw side I.D. so that there is no need to use a drill bit to clean out. Stepper side shaft fitting is a little tighter.
Added ZAxisCouplers_Short in case X axis rail clearance is an issue or if you simply would like smaller couplers.
Version 1: 75%, Version 2: 50%, Shorts 50%