Update: I recommend using this design instead, as it works better with the box turning upside down, etc.: https://www.thingiverse.com/thing:763835

This is a box that only opens after it has been spun counter-clockwise! Any other motion does not allow the box to open.

This requires eight 6mm balls or bearings. I have used airsoft BBs purchased at Walmart (see the last gallery picture). I suppose you could try to print your own 6mm balls too, though I haven't tested that idea.

Inspired by this box:
http://www.instructables.com/id/3D-Printed-Centrifugal-Puzzle-Box-solved-with-/

The box above will open when spun in either direction. My box only opens when spun counter-clockwise (which is not the way most right-handed folks spin things).

This has now been tested and works like a charm!

Video of how it opens:
http://youtu.be/nhxeBcmDFvA

For the full box, print the lid and the box (or the "both" file, which has both, if your printer is big enough). Put a 6mm ball in each of the 8 ball locks, spin it counter clockwise to put the balls in the right spot, and then put the lid on. Shake it up and it shouldn't be able to open until you spin it counter-clockwise again.

I recommend printing the "single lock" versions of the lid and box first (as shown in the gallery pictures), to make sure that the sizing is right. If they've printed correctly, you should be able to put a ball in, put the lid on, then tip it one way and find that it won't open, and then tip it the other way and find that it will open.

No support should be necessary. There are small bridges and 45 degree angle extensions, but nothing that should require support for most printers.

Fool your friends by printing the box, showing it to them, spinning it clockwise, showing them that it won't open, and then letting them try to open it. The clockwise spin will throw them off and make them think that spinning it isn't the solution.