As shown, this screws to the front of the box frame Prusa i3.  I assume it could be modified to fit the aluminum frame unit as well.

This is designed for a #6 screw.  You could use others, just change the nut and screw diameters at the top of the SCAD.  Note that one of the screw diameters is for "friction" - it should be a bit small to keep the screw from creeping around and rattling.

I'm finding this to be more accurate and repeatable than a microswitch, easier to mount and adjust than an optical.  The hall switches I've tried seem to drift at varying ambient temps and over time.

Insert the wires through the holes in the switch carrier bit, solder to the tactile switch, then pull the switch down into the holder.

Put the screw holder onto the X rails as shown, leave 5 to 10 mm of space between the X end and the screw holder for adjustment. Screw the switch holder to the front of the machine so that the switch lines up with the screw and will get pressed somewhere around the right Z height.  You can now tap the screw holder along the X axis so that the screw lines up perfectly with the switch.  Adjust the Z height as needed.