Remixed this design. I liked the size and the hinge from the original design, but wanted to make sure the booklet would stay closed. I extended the case slightly to accommodate 2mm x 6mm rare earth magnets. The depth of the hole was set to 2.5mm. After pressing the magnets in, they are flush and the case closes completely and stays closed. One magnet probably would have been fine to keep it closed, but went with two sets (top and bottom) to keep it balanced and to make sure the corners closed all the way.

Printed it with a brim to make sure it stayed flat. Print flat on the bed, as shown in the first picture.

Model Update 2/5/23:

• Reworked the original model to clean it up
• Add STL for 6x2mm magnets from cleaned up model (ending in _6x2mm)
• Add STL for 8x2mm magnets from cleaned up model (ending in _8x2mm)
• Add cleaned-up Fusion 360 .f3d file
  • User Parameters configured for magnet hole position, depth, and diameter for easy adjustment
• Add new STEP file for 6x2mm magnet holes

Note: I have not printed these updated models, so they are untested, but should not have changed, other than perhaps a slight change in magnet location due to referencing the holes off the left half's edges and then mirroring the holes to the right half.

Find more models at my Printables.com profile
https://www.printables.com/social/513726/models