I designed this to scrub algae in my pico marine aquarium. Each part has a cavity of 13mm x 4mm. It seals in the magnet, so you must pause your print at the right time and insert your magnet. The magnets I used were 13mm x 3.25mm. After printing, clean with denatured alcohol and apply self adhesive velcro (hook to the scrubber, loop to the handle). Make sure you put it on the correct side of the scrubber, since the wrong side will repel the handle. I used Raptor PLA.