Our Printrbot Simple uses 1/4" Acme threaded rod for the Z axis, and came with a piece of vinyl tubing as a coupler to the motor shaft. Sadly, we lost that little piece of tubing, and this printed coupling is our answer to that problem.
The coupler and nut should print easily without support. Depending on the motor you have the fit on the shaft may be tight; you can either drill out the shaft hole slightly, or if you print the part in PLA heat it with a hairdryer so it will slip on.
If you have a flatted shaft one screw will suffice to hold the coupler in place, the second spot is provided for round shaft motors.
I found that the coupling gripped the Acme rod tightly all by itself, but the printed nut is available to lock things together if needed. I recommend finger tightening only.
If you want to tweak the parts in OpenSCAD you'll need the Thread Library for the Acme screw threads: http://www.thingiverse.com/thing:8793