Pangu i3 z end stop holder
This will hold your z end stop firmly in place. I designed this after getting fed up of the included holder occasionally slipping down the z rail.
It attaches to the printer with the existing screw and bolts, so no extra hardware required.
If I were to print this again, I would make it 1cm shorter, as the z axis carriage (that holds the x axis motor) comes within about 1mm of the top.