One additional support should be installed in the center of the structure. This brace is not really necessary to support the platform itself but will keep the masonite overlay to come from sagging. It also keeps the frame from bowing outward in the center and adds rigidity to the assembly when the platform will be in the storage position inside the lower frame.
Cut the brace from a strip of plywood. You can use one of the 2.5 inch wide strips used for the rest of the benchwork, or any scrap at least that wide. The length of the brace is equal to the length of the ends of the frame: 46 1/4 inches.
Use 7x9 inch metal angle brackets to support each end of the brace. Attach the short end of the bracket to the brace and the long end to the center brace of the frame. Pre-drill and fasten with 3/4 inch wood screws in the same method used for the corner brackets.
