I decided to investigate touch plates. Because I have the Shakeoko from Sparkfun with the non-standard controller I was worried about using the “active controllers” like the always-out-of-stock carbide unit.
This fairly recent video seemed to provide a good solution for me (the Sparkfun has two probe screw terminals which accept two wires directly). I don’t see anything in this forum so here is the link:
He seems to be talking about this (now $39):
Note that the seller lists a much more elaborate configuration routine which I think is because instead of just centering on the hole he is finding the x-y edges of the plate(?)
Anyway I am willing to invest in this. I think I may also buy one of those cheap chinese Z plates for which ebay has about 100 listings–for $3 what could go wrong