These are the lego materials that you need: 4 wall pieces, and 2 8x8 stud floor boards. It is preferable to put the safe into a lego building.
Step 1: Building the Safe
Lay one of the floor boards down and put the walls on the floor board. Simply add the other floor board as the roof and you have yourself a lego safe (If you want you can add a door to your safe).
Step 2: Putting the Safe in a Building
Then put some money into your safe, and put it into any lego building (you can add lego guards to "guard" your safe for props)and you can be pretty certain no one is going to find your lego safe. It may be plastic, but it sure is safe.