After repeated failures of the standard z-axis rod coupling, a need for a more robust coupler was determined. This coupler has separate clamps for the 3/16th drive rod and the motor shaft, centering both without stretching the PLA too much. Additionally, it has recesses for the nuts, allowing very easy tightening and adjustments. This design however uses 4 bolts and nuts instead of 2.
The given values are for the SAE Prusa Mendel. You will need to modify settings for metric. Also the screw holes are a bit larger then they need to be, but it worked great for me. If your layer thickness is 0.4 or greater, you may need to increase the size of the bridging layer. You will probably also want to saw off excess screw after tightening around your rods, as the 1" #6 screws will be too long.