Based on kart5a's spiffy design. I found the original to be a little weak across the coin slot, so I've thickened and deepened the design. I also radiused the slot to reduce stress risers, made the magnet slot completely enclosed, and added a slot to hold caps for photographs. If you want to use a magnet, you will need to drop it in while it's printing. (And depending on your printer, might need to use another magnet on the outside to hold it in place so your print head - or in my case, auto-level probe - doesn't pull the magnet right back out again.)