As seen in season 3 episode 9 The ABC's of Beth and the season 4 Thanksgiving YouTube teaser. The magnet is printed in two parts that snap fit together. You can add some glue if you want. Currently it holds a 2mm thick 10mm diameter magnet*, but I will add some extra sizes later as well.
Magnet in the pictures is printed in Colorfabb nGen (silver) and Das Filament PLA (red) on the Creality CR-10.
*I always order my magnets from Supermagnete. They always come with gummy bears [which I know 3D printer people like :)], some free other magnets and a thank you card with the whole company including the dogs on it. How can you order somewhere else? :D