"Help Keep The Beach Clean, Please Take Your Trash With You"
A few years ago some brands made ads with objects similar to this, they were very useful to keep the beach clean.
Magnets 3x18mm (chinese) + epoxy glue (some tolerance)
added 2 files for development where you can invent a lock system to avoid magnets:
filament PM PETG
Nozzle 0.4mm - 2 walls 100% infill @ 235ºC / 80ºC 40mm/s, fans @ 50%