The diameter of the holes is somewhat flexible, but I was thinking in the neighbourhood of about 8 inches dia.
Diameter of a puck is 3 inches by 1 inches height.
I would rather not doctor the pucks...meant to be used on outdoor rink...pucks get lost easily in the snow despite backstops... about 50 pucks get used throughout the winter. If this is relatively easily done with cheap magnets, sure I am willing to drill 50 holes and backfill with silicone or somesuch.
Some sort of active sensor like the ones protecting us from being squashed by our garage door (opener) cannot be cheaply purchased/incorporated? With the configuration in the images, this would "only" require 3 sensors - if the middle hole is removed or lowered to the bottom row, 2 sensors?