This is a cat collar pendant that works with our magnet-activated cat door. You can buy replacements for about 5 bucks each plus shipping, or print your own for about a dollar each.
It is designed to captively contain two 1/4" dia. x 2/10" thick magnets. The print is paused at z-position ~12mm, the magnets are inserted, and the print is allowed to finish.
My source is K&J Magnetics, and this is the part number:
Note in one of the images I uploaded that there's no notch in the file, I just included the notch so you can see inside that there's a cavity for the magnets.
Print with a raft since this is a pretty tiny surface area. Using Jetty or other firmware upgrade, set the z-position pause at around 12mm. Insert the magnets, resume the print.
I'm using a Thing-o-matic with a mk7 extruder. So the two closest metals during the print are brass and aluminum, neither of which are affected by the magnets.