Summary

This is a trick opening box!
Following the proper steps, you will be able to reopen it.

I've added some clues on how the puzzle work in my blog: http://wp.me/p3gI7M-4V

Note: You need two 1/2" (12.7mm) diameter marbles or ball bearing to operate the mechanism inside.

Instructions

This puzzle is clearly more complex than my previous one. To get the best feeling the pieces have to be tight, so I've reduced the clearance to 0.5mm in the sliding areas.

Print the following parts:

1x box
1x Door
2x Side Panel
2x Keys
2x Marbles (optional)

Look the second picture for assembly instructions.

I've tried the reduce the over-hangs on most of the pieces to ease the print. It's still a "work in progress" as I couldn't print it yet and I have a concern on the durability of the 2 top notches in the box piece to slide the door. They may break if care is not taken while shutting the box.

I have to thanks Ttsalo for his idea of springs using a rod. In a sense this box is a sort of padlock where the key is replaced by the marbles.

I've included the STL for the 1/2" marbles, but I would recommend to use a glass or a ball-bearing.

Keep me updated if you try to print this!

Double Keys Box Puzzle