These pieces go on the Y-axis end plate, preferably the one to which you've already attached your controller, so that the wiring is minimal.

One bolts to the inside of the end plate so that the X-axis plate will hit the switch before it hits anything else. Wires come back through the tiny open bits of slot next to the makerslide ends.

Second has a slot in it to fit the angled edge of the end plate. You will need to bolt some kind of spacer onto the rail end plate to meet it.

All in openscad so you can adjust for the size of your microswitches or the tolerance of your 3D printer.

UPDATE: I've fixed the Y-axis stop so the microswitch lines up with the hole in the long-rail end plate. Pretty much any kind of bolt in there should trip it. Also added a Z-axis stop setup consisting of a holder for a switch that sits on the bed and an adjustable probe that bolts to the Z carriage next to the spindle holders.