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*. I might add some extra sizes later as well. Otherwise, feel free to remix!
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 the 3D printing community likes :)], some free other magnets and a thank you card with the whole company including the dogs on it. How can you order from somewhere else?! :D