For the box, just add supports to the outside and print. Make sure that there are no supports inside the hinges. Once printed, force the box open and closed multiple times to loosen the hinge action. I found that filing the the contact area between the latch and contact bar helps opening the box again easier. For the corners, I printed it with a brim. Just glue them to the box using regular super glue.

Check out my short video of the box on my Instagram page: https://www.instagram.com/p/BbhtaZ8nwCR/

Link to the Harry Potter cufflinks: https://www.thingiverse.com/thing:2663448