Rip a sheet of 3/4-inch plywood into two pieces that are 18 inches high and as long as each corresponding cleat. These will act as the structural faces of your bench seating. Cut two pieces of plywood to 16" x 18" and set them aside for later. Slide the appropriate plywood face through the 3/4" gap in the cleats (Image 1). Notch out the corner of this piece so it will fit snugly in place.
Hold up the other piece of plywood in front of the other cleat. Square up the two pieces of plywood and drill through the "slide through" face into the edge of the other piece (Image 2). Span a level across the top of the plywood fronts and the 2x3s on the wall, and shim, if necessary (Image 3). Once the faces are level, use a screw gun to fasten them to the cleat (Image 4).