This is the print-in-place 30-card sample box that I've been using.

The print profile has a pause to insert four magnets. Make sure to pay attention to the polarity of the magnets so they will stick together when closed.

The magnet opening is 8x4mm in size. The original instructions say to glue 6x3mm magnets. I'm sure this is fine. I have 8x2mm magnets on hand so I put two in each spot.