Cut one edge of the box and open it flat on a flat surface. Measure the costume wearer from side to side across the front and front-to-back along their side. Measure the dimensions for your costume wearer on the box and mark the lines for folds with a yard stick and marker. The sides should be slightly bigger than the measurements of the costume wearer for comfort. Use the template for a guideline, but be sure to cut the sides deep enough for it to be comfortable under their arms.
Paint the box with black craft paint.
Cut two strips of cardboard 2-1/2 inches wide and the length to match the width of the side plus 4 inches. Fold the ends of these strips over at 2 inches on both ends. Paint the surface of these strips with black craft paint. Attach the flaps on the strips to the inside of the top of the costume with a hot-glue gun, positioned close to the sides.
Cut a circle from the red craft foam. Cut three rectangles and two smaller squares from the gray craft foam. Cut 12 ovals or circles from the gray craft foam for the number buttons. Lay the costume on a flat surface with the front facing upwards. Arrange the shapes on the front of the costume and then glue them in place with a glue stick.