I got a set of 18mm circular magnets from the art store today (3mm deep) so I could make a set of magnets for our fridge. As an easy first magnet, I modified my design for the Majora's Mask Necklace by removing the necklace loop and cutting a hole in the back for the magnet.
Support will most likely be needed for the hole. It was a pain to get out because the hole is fairly shallow, but it was definitely worth it. The magnet fit perfectly, and is holding in by friction.