One of the side-effects of adding bed risers to my CNC is that I can pass the front edge of the bed slightly, which makes it a convenient place to mount a microswitch to use as a tool length offset probe. This mounts to the front, slightly high to allow for a printed sacrificial bed and PCB bracket, with some wiring channels off either side. It's wired in parallel with the Z probe (A5 on a typical GRBL board).

For tool changes in software this works perfectly for milling PCB's with a combination of pcb2gcode and bCNC; YMMV with other stuff.