A rotatable box that looks similar to the classic woodworking “impossible” dovetail but is opened via rotation instead of sliding, allowing the box to have a much larger internal cavity (over 1500 cc) that can be used to hold gifts, etc.  This model also includes an optional locking mechanism utilizing ¼" diameter by 1/16" thick neodymium disc magnets and 7/16" diameter ball bearings.

This looks best when the two parts are printed in different colors.  The holes for the locking mechanism magnets are sized to press fit ¼" diameter by 1/16" thick magnets.  If the magnets are loose just use a little cyanoacrylate glue.  Smaller diameter (6 mm) magnets can also be used by gluing.  The holes for the locking bearings are sized for 7/16" diameter but smaller (10 mm) bearings should also work.  You only need one bearing and two magnets but the model has holes for up to four bearings and eight magnets.

Locking mechanism works best by holding the model in one hand and bumping it into the palm of your other hand.  Hold one side up to lock, the opposite side up to unlock.

No supports necessary.  Clearances are fairly tight so your X and Y dimensions need to be reasonably similar.  Fit seemed about right when printed on my Prusa i3 MK3S.  I used PLA but PETG and other materials would probably also work.  Design comes from the January, 2010, Inventor NG World Cup challenge, idea by T. Hutterer.