The end stop takes a pressed/dropped in M3 nut. An M3 screw with another nut on it (to lock it in place) screws into the pressed in nut. You may wish to drill the screw hole all the way through to the motor mounting screw hole to give yourself more adjustment room; the model didn't cooperate with me!
FYI, JM's plan was to use a spring to clamp the Z nut against the top side of the nut slot (aiding gravity), rather than as an anti-backlash nut. Using the spring allows a little bit of angular misalignment tolerance in the system. The original system could be very tight if the nut was tipped a little bit in the slot.
**** After installation ****
JM's nut system works very smoothly! Kudos! The linear bearing system is much easier to install and adjust. The triple bearing carriage doesn't have any visible slop and is running very well. JM's new tensioning scheme works well.
FYI, if you add superglue the belt is REALLY hard to get out, but it won't slip. Don't clamp the second belt end until you have your Z shaft spacing set. Once that is done, move the motor as far as possible toward the idler and tighten a screw. Pull the belt tight, then go looser until the notches line up (there are pictures of the belt tension before and after moving the motor). Press in the belt, leaving it centered in the belt holes in the X ends. Loosen the motor mounting screws and move the motor away from the idler. Run the carriage back and forth to make sure it runs smoothly. Tighten the screw on the idler side first, then the others. Re-check for smooth running!
**********
Latest update: my Z adjuster broke and I designed a beefier version. I also added tabs to attach my wires to. The blue picture is the latest version. You don't need to superglue the belt into the teeth on this version!