edit/ Added SCAD file /edit
Based on http://www.thingiverse.com/thing:23038, but with retained nut, and clearance for high-Z.
TheGrum's version was fine, and I liked the Printrbot logo.
A while ago I was learning OpenSCAD so decided to do a version for 8mm threaded rod and with the nut retained (also has no bridging). See the v3 stl & the red one in the photos.
It did improve the smoothness of the prints. See the close-up, the red one was printed without a joinder, the white one with the red fitted, much crisper edge, but I found the stepper crashed into it at high Z.
So I did another design moved back on the Y axis, see the v4 stl, the white ones in the photos.
Remember it is NOT a handle.
Print a little chunky (I did 2 perimiters & 20%), my first print(at 1 perimiter &10% fill) split 1/2 way up the threaded rod level when I forced the rod.
I had to drill out a small amount in the 8mm holes, the nut trap was fine for the nuts I have.
Insert the top threaded rod into a drill chuck to easily move them into place.
Move Z high so the ends are in the right place (in case they are slightly misaligned). Mount one end on the Z rod, then align the other by spinning it until it lines up with the other Z rod.
Consider whether you want the threaded rod longer to potentially mount other things.