Remix of Tom_Maragos's http://www.thingiverse.com/thing:104733 to fit my V1 Printrbot Plus.
The OpenSCAD file takes the original STL, closes the hex nut and threaded rod hole, creates new centered rod and hex holes (joined with a conical section to avoid bridges during printing.
This is set up for a M8x1.25 metric rod and nut, but the OpenSCAD file could be easily changed for any size.
The original file has differing left and right units. I used the same unit for both sides, but had to trim a tiny bit off the left unit with an x-acto to accommodate the z-axis end stop holder.
It's tight on purpose and needed a little trimming to fit the z nut. This is good, should eliminate all but the nut/rod backlash.