16 pcs of 3mm x 6mm diameter magnets

Make sure all magnets are facing the same way.

Press fit into holes, you can use glue after to hold them in better.

You might need to glue two inner halves together.

You can iron top surfaces to be smoother but this creates more static friction.

Comment if you'd like me to upload versions that can accept different magnet sizes.

Updates:
Added filets to some edges