This is a very simple box design using a small box (a shoe box or smaller works) with a simple lock mechanism to represent the opening of the box. When you get the code right, a green LED lights up and the servo moves, unlocking the box (or at least making it easier to open). Otherwise, a red LED is lit up and the box remains locked. The default code is, sequentially, starting with the button attached from pin 12 and going down to the button attached to pin 9.
Build a box with a keypad-like lock.