Or if you bump it or remove a really strong magnet…
Jigs really are your best option.
Not sure what your coasters are made of but if you aren’t cutting them yourself beware that they might not be square. I posted about jigging irregular coasters here:
https://community.glowforge.com/t/alignment-of-irregular-objects-coasters/16116?u=evansd2&source_topic_id=18395