The idea came from http://www.thingiverse.com/thing:65997 which was too small for the PrintrBot Plus 12mm z rods. If your printer settings are out of wack you may have to (SLOWLY) drill out the rod holes. I find a rotozip works best at slow speeds. Take your time and it'll come out well.
This thing was made with Tinkercad. Edit it online https://tinkercad.com/things/2xI2G0z2rQb
Updated STL to the new version that uses less plastic. I
Print two, clean, and put on your Z rods. The slot on top is big enough for a 12 mm rod or dowel. This dowl needs to be sturdy enough to support your filement. You may want to put some clamps on your rod to keep it from sliding out. I find that binder clips (like used to hold the glass on the bed) work great.
You may need to drill out the holes depending on printer settings/alignment. I use a roto zip (actually a dremel with a rotozip attachment) without the guide. This cracks the plastic less, and if you use a low speed, it's very easy to get a pro looking job.