A small and handy safe box with a built in combination lock. The whole box prints preassembled (inspired by emmett) including hinges, the only thing to add are the combination lock dials and rods which easily slide into position.
Two hexadecimal dials ensure a completely safe solution with 256 different possible keys (that is about as safe as WEP). Choose your own favourite key!
Print at fine resolution. Carefully push the two box halves to close the box until the hinge breaks free of any stuck plastic and moves freely.
Print two dials and two rods and place one dial on each rod. The flat surface on the rod closest to the dial defines the lock open position - choose this to match your favourite code. There is actually a small 16-facet part of the rod (yes, very close to completely round) and this is where the dial should be (the inside of which is also 16-facet).
Do not forget the correct key. It will probably take you up to 60 seconds to retrieve it otherwise.