Tape the cardboard box shut. Lay the box on its side. Place a utility knife about four inches from the bottom, through the cardboard, on the side parallel to side taped shut. Cut moving upward about three feet, depending on the size of the box. Turn the knife and make a three-foot horizontal line. Turn the knife again and continue cutting vertically until reaching four inches up from the bottom. This is the fort's door. Cut a 1-inch circle, which serves as the password opening, near the door. Write "Say Password Here" next to it.