Rip two strips 15 1/2 inches wide and 8 feet long with a circular saw from the 3/4-inch thick plywood sheet. Cut panels 70 inches long from each strip. These will be the top and bottom of the box. Cut 17-inch-long panels from the remaining part of the strips. These will form the two sides of the box. Cut the remaining 17-inch-wide strip to a 71 1/2-inch long panel. This will form the rear of the box. Cut the remaining piece to a 15 1/2-inch by 15 1/2-inch panel. This will form the center support.
Measure and mark a horizontal line 1 1/2-inches from the bottom of the two side panels and the rear panel. Drill two holes with a spade bit between the marked line and the bottom, 4 inches from each end of the three panels. Cut along the line between the two holes as well as down to the edge. The remaining part will form the legs of the bench.
Screw and glue the bottom panel to the rear panel 15 1/2-inches from the top. Screw and glue the top panel to the top of the rear panel. Screw and glue the side panels to the rear panel in between the top and bottom panels. Screw and glue the center support in the center of the box, in between the top and bottom panels.
Screw and glue a 70-inch-long length of 1-inch by 1 1/2-inch lumber under the bottom panel at the front to form the kick-stop.
Cut 45-degree angles at the ends of two 70-inch and two 15 1/2-inch-long lengths of 1-inch by 1 1/2-inch lumber (commonly called 1-by-2 lumber). Screw and glue these lengths of lumber to the outer edges of the box to form a frame. Screw and glue a 15 1/2-inch length of 1-inch by 1 1/2-inch lumber to the center support.
Cut and screw a 3-inch-wide and 70-inch long strip of plywood to the top panel rear edge, overlapping the outside rear edge by 1 inch. Screw and glue a 3-inch-wide strip of 7-inch-long plywood to both sides of the top panel, overlapping the outside rear edge by 1-inch. This three-sided frame will form a support for a seating cushion for the top if you decide to add one.
Paint the entire mud bench.