I was experiencing z-wobble with the existing coupler on the printrbot simple so I designed my own. I have yet to test it, but it should work. This is shorter than the original printrbot coupling, so should not interfere with y-axis motor.
the tube used to make the flexible coupler on the Simple is taken off the screw and inserted into the smaller side of the coupler. The lead screw is then inserted into the other end. The whole coupler is secured using 4 M3 x 20 screws and 4 M3 nuts. In order for the extruder to come into contact with the bed, longer springs need to be used along with 4 M3 X 30 screws. If you don't have longer springs, simply print up some M3 spacers.