Here are all the materials we need for the final assembly: our two box halves, our ½" ball bearing (which fits beautifully), four small magnets, and a little bit of super glue.
Carefully squeeze a few drops of super glue down into the magnet pockets in each of the two halves...
...then carefully place a stack of two magnets into the pocket...
...and repeat for the other side. Be sure to let the super glue set for at least 20 minutes, or else the ball bearing may pull the magnets out.
After the glue has set, place the ball bearing in the deeper of the two pockets and then slide the two halves together diagonally as shown above. To lock the box, hit the top of the box to release the marble from the upper magnet and onto the lower magnet.
The completed box! Finish the edges with a small handheld file as needed.