This is quite probably the most complicated and largest project I've taken on to date; a jewellery/trinket box with a magnetically-locked door. The hinged door has a cog which rotates into place, then when the gold tab is moved to the left, locking the cog in place so the door can't be opened. You can then even remove the locking tab and put it somewhere else and the door won't be able to unlocked. You'll need to source two 20mm x 10mm x 2mm neodymium magnets, one for the locking tab and one for the lock mechanism.
If you don't have a 3D printer but you'd really like one of these please send me a DM and we can arrange for me to produce one for you, or check out my Numonday store:
Or on my eBay page: 'enterprisexddesign2016' (formal shop to come)