1. Assemble the box: Glue the bottom flaps in place, then glue the top flaps down against the inside. Place heavy books on each side for about half an hour while the glue dries.
2. Measure the box, then add ¼ inch to all the measurements. Add a ½-inch inseam all the way around the edges, and 4 inches height (for overlap) to each side. To use just one piece of fabric to cover the sides and bottom, your drawing of the piece will look like a plus sign. (Click the sketch to see it larger.)