Place the 58 inch ply piece on your work surface—this is the base and will make a two person seat box. Run a line of glue along the 30 inch edges of the base, then position the 30 inch edges of the two 30 by 35 ply pieces at right angles to the base, one on either end, clamping them in position. Let dry.
Hammer a nail every two inches through the glued joint of the two 30 inch sides, then take off the clamps.
Run a line of glue along the exposed, vertical 35 inch sides of the attached box ends, and the horizontal exposed sides of the base. Position the two 60 inch pieces of ply, for the box sides, so they are flush with the side end pieces and the base, forming a rectangular box. Clamp them in position and let dry.
Hammer a nail every two inches through the glued joints between the sides of the box and the end pieces and between the sides of the box and the base. Take off the clamps, then fill any exposed nail holes with wood putty, scraping the putty on top of the nail head, leaving an even surface with the nails disguised. Let dry.
Position the piano hinge—a long, continuous hinge piece—centered on one edge of the remaining 62 inch ply piece, the lid of the box. Screw the hinge in place along the edge of the lid, then position the lid over the box, again, making sure it’s centered, before screwing it into one side edge of the box. You will now have a box seat with a lid that lifts and has a one inch lip over the sides and front of the box.
Sand the box thoroughly, paying special attention to the lid, rounding off any sharp edges on the lip of the lid. Paint or varnish as desired, then let dry before positioning in your chosen location.