I use the LJ12A3-4-Z/BX proximity sensors. You can run them off 24v (10-36v) and the wiring is simple. They are Normally Open switches, so you can put as many as you like with the outputs all connected to one active low input on the BoB. They are specced as 6-36v but don't switch reliably below 10v. Brown wire is to supply +ve, Blue to GND and the Black wire switches to GND when Ferrous metal is within 4mm of the sensor head. Again I say, mount the sensor so that the metal passes by it not end on, cause it will get crushed.

Cheers,

Rob