Kossel 2020 OpenBeam Endstop

Inspired by [boofly](http://www.thingiverse.com/boofly)'s [endstop for 1515 beams](http://www.thingiverse.com/thing:609791), I wanted to put together something similar for my delta printer, which is built with 2020 openbeam.

The advantage of this is that you can adjust the position easily and tighten it down from the outside. However, it's more susceptible to minor changes in the angle of mounting. And your mileage may vary regarding the horizontal alignment of the endstop switch.