I created these to upgrade my FolgerTech Kossel to Optical End-stops.
They mount right to the 2020 extrusion using a M3-8mm or M3-10mm screw and drop-in T-Nut.
I printed these in PLA with a .4MM nozzle and .3MM layer height at 60% fill, but I'm sure other settings would work well too.
The Opto mounts to it with a couple of M3-10MM screws threaded right into the plastic. (Don't tighten them too much or you'll start to strip the hole, just snug them up.)
It also works to hold the standard microswitch end-stops, but you can't access the adjustment/mount screw with them mounted.
This thing was made with Tinkercad. Edit it online https://www.tinkercad.com/things/5rhAJ0kUdwv