Print a lock mechanism (mechanism body, bolt, tumblers and key) from the printable padlock item thingiverse.com/thing:27205 and assemble it using the instructions there.

Print the box and a lid, there's a solid lid and a slotted one for use as a piggy bank. Insert the mechanism into the compartment inside the box, bolt pointing upwards, slide the lid into place and turn the key 180 degrees counterclockwise to lock the box.

The scad file contains in addition to the box, all the code for the lock mechanism and the padlock as well, so if you want custom keying (or more tumblers or a bigger box or something else), you can modify the file in this thing and export the STLs.