This is a classic wood working trick.
A dovetail is usually used to joint two pieces of wood together.
This box is a "modern" printable version of it, print it and let your friends figure out how to open the box.
For the locking mechanism you need two round neodym magnets 2mm thick and 10 mm diameter, and a 10 mm steel ball.
The video shows (SPOILER ALERT) how to open the box by taping it against a hard surface, this makes the steel ball "swap" magnet and get out of the way so you can slide the two pieces apart.
This is by no means my idea since this has been around for many many years, this is just my version of it with a small compartment in it and a hidden locking mechanism.
No support needed, all tilted angles are 45 deg, most printers should handle this without issues.
2x round neodym magnets 10 mm diameter, 2 mm thick.
1x steel ball 10mm diameter
Glue a magnet in the bottom of the smaller hole in each piece, place the steel ball in the deeper hole, then slide the two pieces together, then smack the box against a flat surface so the ball moves to the other magnet, it now locks the two box pieces from sliding apart.