Someone who doesn't know how to open it will struggle for a long time, especially if you give them no clues. Once you've learnt the gesture, you can open the box in a couple of seconds. My uncle (who is a maker working at a museum) took 24 hours to open it, with a team of interested buddies, an xray machine, a metal detector and large magnets!
Many thanks to London Hackspace, and everyone who has helped me out with learning how to use the laser cutter! Thanks Lou for the good photos! This design has taught me a lot about using the laser cutter, parametric design and designing for CNC. This is about the 12th version, so it's pretty reliable by now. I hope the instructions are good enough!
Tools and Materials
Tools: a laser cutter, a CAD program
Materials: a sheet of 4mm ply, PVA glue, a small rare earth magnet, 2 x 25x4mm springs (you can get them from pens), a 4mm ball bearing and 2 x 10mm of 4mm metal rod (I use brass).