You can use this with or without bearings. Minimally, you will need two printed towers and a ~8mm rod. The regular rectangular tower is easier to print vertically, but will restrict the range on the z-axis on the power-cable side of the printer due to the extruder motor, so I created a tapered version as well.
There are small endcaps included that work (tightly) with a 5/16 threaded rod and keep it from coming off the towers. Use these or something similar to keep the rod from sliding out.
If you use bearings, just slide them on and rest them in place in the top center of the towers. If you don't use bearings, just set the rod into the V-slots. No tools necessary for spool changes either way - just carefully lift the rod from the tower and make the switch.
Additionally, my printrbot+'s (v2) wrapped cabling kept getting caught on one of the thumbscrews for the bed leveler when the platform moved in the Y direction, so the towers each have a 45 degree 8mm square hole to hold accessories, and I included an optional simple hook that I used to suspend the cables via rubberbands above the print bed.
This should work for anything with exposed 12mm Z rods like the printrbot+, but I've only tried it with mine.
Update Added another 'accessory': a Maglite clip for 18mm flashlights. Slice with supports for printing. It's not perfect since the extruder and fan assembly casts a shadow, but it helps out. Make sure your towers are on the Z rods far enough to be secure.