At the time I simply used some aluminum tape on the side of the bowl as one side of a ‘probe’ and an alligator clip to the router bit. I have mentioned that I use Estlcam programmed onto an Arduino Uno for simplicity. The probe leads plug into that. It’s a simple NO circuit.
I have since abandoned that approach for a much better one using Grblgru, Grbl-Mega-5X and an Arduino Mega 2560 board +/- a RAMPS board. The probe function and subsequent ease of making an STL from it are far easier. I made a very simple Z probe for that using a limit switch, a bit of wood, some wire, a pen spring and a couple of rivet heads ha ha. I posted a picture of that in this forum.
It is still working just fine.