The z endtop on Printrbot LC v1 is pretty annoying. The assembly videos dont show the endstop being placed, its simply explained that it needs to be glued on. At first the wood held the scre in place just fine, but with continued adjustments, the wood wore out, and I had to add a nut to keep the screw from falling through.
Originally I had just glued the top nut in place with wood glue, and had not bottom nut. Then the vibrations from the printer made the screw slowly turn, so I added a screw at the bottom to tighten it. However, this meant that every time I needed to adjust the screw, I need to stick my wrench or some pliers back there to hold the bottom nut.
This simple little print solves the problem once and for all.