Stitch with right sides together, creating a box, but leaving an opening to insert foam. Cut four pieces of fabric 37″ wide x 19″, four pieces 19″ x 5″ and four pieces 37″ x 5″.
Support the hinges with a strap to keep the cushion from collapsing all the way forward. Flip it back over and attach the cushion with hinges. Then flip over and screw the base to the sofa underside with 3″ screws. Start by screwing the storage box to the back, through the 2×10 back boards into the supports in the back of the sofa.If you choose to use 2x4s for the legs, remember, this does change the dimensions of the legs, and adjust the 2×2 cuts accordingly. The 2×2 aprons are to keep the fabric tucked under and to give you something to screw your feet too. Reference the diagram in step 6 for exact measurements. Turn under and staple to the underside of the legs. Anyway, cut out the side and back pieces and stitch together, and fit over the back. To make this part easier, you could simply not put a taper in the back side of the sofa, and all your cuts are square.
Now cut strips of fabric as shown in the diagram above (step 6) and sew together and staple in place around the sides and front of the sofa. Place the cushion slipcover over top of the foam and staple the slip cover raw edges to the underside of the plywood, hiding edges. Sew together to create a box cushion (see this post for details on sewing). Cut a piece of fabric 73″ long x 31″ wide. Then use the spray adhesive to attach batting to the foam on the top, front and sides.
I did not wrap the foam, simply cutting it 9 1/2″ tall, surrounding the front and sides of the sofa. Then screw the plywood to the bottom as shown above. Read the direction on the can of the spray adhesive for the best adhesion. Then use spray adhesive to glue the batting to the back as shown above.
You could also use store bought sofa feet. The legs above are shown as 2x2s, which you could do but a center fifth leg is recommended. It is after all made from a standard camping pad. and did anyone notice the little hinges on the front seams?
Also, the deep cushions are perfect for sleeping on. Instantly, the wheels were turning in my head. Based off a sleeping pad foam cushion, so seating surface doubles as a guest bed.