Line up four 17-by-17-inch storage cubes side by side in a row, with all the cubbies facing the same way.
Apply wood glue on the sides of the cubbies that touch each other then press the cubbies together so you have a long row. Repeat with another four cubbies.
Position the two long cubby rows facing back to back, with 17 inches between them. Insert a single storage cubby facing outward between the ends of the two long cubby rows to form a rectangle of cubbies. Glue the single cubbies into position.
Paint two sheets of 3/4-inch plywood or 1-inch MDF that measure 27-by-75 inches in the same color as the cubbies and let them dry.
Position the plywood or MDF sheets over the top of the rectangular base of cubbies to form the platform top of the double bed. There will be a slight overhang on all sides.
Secure the platform in place with 1-1/4 inch screws through the top of the platform around the perimeter into the cubbies underneath every 4 to 5 inches. Drive a second row of screws about 15 inches in from the first perimeter row to finish your double bed storage cube.