Line up the fabric for the exterior of your box with the fusible interfacing and iron with a hot iron until fused. Cut a 4-inch by 4-inch square from each corner of both the interior fabric and the exterior fabric and interfacing.
Lay the exterior fabric and interfacing face up on your work surface. You will see four "arms." Fold each arm up to the one next to it, and stitch the seams together to form a box. Sew the interior fabric using the same method.
Turn the box made from the exterior fabric right side out and put it inside the interior box. Push the corners out and make sure they are correctly aligned. Stitch around the top of the boxes, 1/4-inch from the edge. Leave at least 3 inches so you can turn the box the correct way.
Pull the fabric through the gap left until the right sides of the box are facing out. Close the remainder of the seam.