UPDATE Dec 3 2016: My end stop cracked where the bolt tightened onto the conduit, so I made an updated version that is thicker and stronger. Also fixed a problem with the IE version where the bolt hole was in the wrong position.
NOTE FOR MPCNC 525: This was designed to support the original MPCNC. I expect the end stop holders themselves to work fine on the 525, but the light barriers likely don't mount on the new design. I plan on upgrading my MPCNC and plan to update this design once I have the CNC working.
Remixed Allted's end stop mount for the Mostly Printed CNC to work with an optical end stop. This design has two end stop sizes, one for X and one for Y ("Short" and "Long"). I place these on the outer rails instead of the moving X and Y to keep the wires from moving. There is also a small opening to allow for a wire wrap to help keep the wires clean. These are clamped on with the same #6-32 3/4" bolt and nut that are commonly used in the MPCNC. Note that these are designed to be min end stops for X and Y, and will not work as max end stops without further refinement.
Also included are two light barriers to intersect with the optical sensors. One of these (the "Bolt On" version) is designed to use the existing bolts on the "Roller_F" unit (though you may need longer bolts, but I've tried to keep the material thin so it can just squeeze on), and the other (the "Screw In" version) is for directly screwing onto the "Roller_Lock" (which doesn't have any hardware on it to use). For this, I use a couple of short (1/4 to 3/8) #4 pan-head screws and pre-drill the holes.
You likely will need to print four files - one short and one long end stop (both US and IE are included), and one of each of the light barriers. See my pics to get an idea of how I am using these.
I designed this to be printed without any supports. There is very little infill used, but 20% works just fine.