The ones I used are no longer available but were similar to these:
I specifically used a normally closed setup so if there was ever a failure of the wiring it would cause the machine to stop rather than crash into the end stops. Because they required 12-36v and the Onefinity controller operates at 3.3v I initially used a relay board I had between them but eventually moved to an optoisolator once I could get one to avoid the mechanical nature of a relay - both methods proved to be equally as accurate. Under normal operation the relay would be held closed by the normally closed proximity sensor and would open during the homing cycle which would break the circuit on the 3.3v side to the DB25 breakout board.
The basics of the mount looked like this, note the metal screw in the plate for the proximity sensor to detect. I can dig up the stl file for the mount, same one works on X and Y. I don’t have one for the Z, that was more “custom” and not something I’m proud of but it works 
