G2S Optical Endstop Mount

This is a mount for a commonly available optical endstop, designed for use with my [G2S 3-Bearing Carriage](http://www.thingiverse.com/thing:1620524), which has its endstop screw centered between the vertical rods. (This screw on the stock carriage is offset by about 4mm.)

I have included an alternative design without the square alignment pegs, which will allow mounting onto any flat surface using a single M3 screw.