Summary

This was designed specifically for the DIY Tech Shop Prusa I3X. There are many adjustable Z endstops, but I wanted one that fit perfectly and was completely reversible. This works perfectly.

Instructions

For this thing you must also supply an endstop holder to match your particular endstop. For the DIY Tech Shop version, this endstop holder was used by me https://www.thingiverse.com/thing:13353. It has two mounting holes, I drilled 2 more so the switch can be mounted in any direction.

Print off the part I made. You will need to supply the following hardware.
1 3x20mm socket cap wrap threads with 1 layer teflon tape wrapped on it
1 3x30mm socket cap
1 3mm washer
1 aluminum spacer 1/4 x 1 inside threads for #6 - cut down length to 20mm - purchased at local hardware store
1 3mm nut

Note: The aluminum spacer has threads inside, but they are not necessary. In this case the 3mm goes thru the threads. If you have a suitable spacer withouth threads it can be used as well.

The picture above shows all the parts. The nut is already installed in the nut trap above. You will need to ream the holes slightly and the nut needs a bit of friendly persuasion, but I wanted the nut to be tight and not move.

Then you assemble. The spacer get pushed in the larger hole, flat with the face opposite the nut trap. Then screw in the 3x20mm about 1/2 way into the nut in the trap. Remove socket cap from the lower right of the x motor mount. Use a 3x30mm and 3mm washer and put the screw thru the aluminum spacer, push the spacer in the motor mount opening, and screw the 3x30mm securely. Ensure it is vertical.

Position your endstop below this, so that the screw head contacts the switch.

Now you simply adjust your z level by turning the screw. The teflon tape provides enough drag that the screw will stay where it is adjusted. This will allow you to very precisely adjust the Z axis.
Enjoy

Custom Adjustable Z Endstop for Prusa I3X or I3XL