A holder for the bolts that holds the Z endstop screw in place on the Printrbot Jr. The screw kept loosening and the bottom bolt rolled down, so I made this little holder ting. Seems to work just fine (although it could - should - be tighter around the wood). Also eliminates the need for a wrench when adjusting the screw.
Welle417 made a tighter version - you might want to print that in stead: http://www.thingiverse.com/thing:82896
I printed PLA at 30% infill, which worked...
1) Adjust the two bolts so they have a flat side parallel with the long side of the wood they are "clamping".
2) Push the printed thing over the two bolts so it's as tight as possible.
3) Readjust the z endstop screw, if needed.
It isn't 100% tight around the wood, but works fine for me. If it's too loose, try pushing it farther in toward the bolt. As an alternative a little piece of paper wedged between the thing and the wood should make it stick better. I didn't need to do it, but I have a feeling each printrbot is a little different from the others...