Lay a 2-by-4 under the left side of a plywood square so the left edges line-up evenly. The2-by-4 will add stability and strength to the ottoman, and will face the inside of the cube.
Drill a screw into the upper left-hand corner of plywood and into the accompanying 2-by-4. Drill an additional screw under the first one at every 4 inches.
Repeat the first two steps with three more pieces of plywood and 2-by-4s.
Join the four pieces of wood you just worked with, using a screw, so they form the shape of a cube. Stand one of the pieces of plywood parallel to your body so the attached 2-by-4 faces your left arm. Then, stand another piece of plywood perpendicular to your body so the side with the attached 2-by-4 is on the side closest to piece of plywood you first stood up and creates a right-angle. Drill the two pieces together with four screws evenly placed apart. Join the remaining two pieces of plywood with the attached 2-by-4s in the same fashion, and drill the first piece you stood up with the last to complete the cube shape.
Attach the fifth piece of plywood to one of the openings of the cube with a screw in each corner.
Stand the cube so the bottom end is on the floor.
Place the foam on top of the cube.
Cut a 32-inch square piece of the upholstery fabric and center it over the foam on the cube.
Use a staple gun to staple the ends of fabric to the wooden cube, tightly over the fabric.
With the remaining fabric, cut a rectangle that measures 65- by 17-inches.
Sew the two short ends of the rectangle together, right sides facing each other, using a 1/2-inch seam.
Keep the fabric rectangle you just sewed with the right sides facing in and slip it over the top of the ottoman so 1/2 inch of fabric covers the top edge ottoman. The seam you sewed should be at one of the cube's corners.
Use a staple gun to staple the large rectangle of fabric around the top of the ottoman.
Stand the ottoman so its open area is on the floor, and pull the fabric you just stapled down so it covers the staples and the rest of the cube.
Turn the ottoman upside down and staple the remaining 1/2-inch of fabric to the underside of the ottoman so staples are out of sight.