Arrange two 1/2-by-36-by-48-inch boards on the working surface, parallel to each other with their long, thin sides on the working surface. Push the boards, which are the front and rear sides of the banquette, until they are two feet apart. Set a 1/2-by-36-by-24-inch board against one set of the side board ends. Align the edges so that they are even with each other.
Make a hole into the new board, which is the side board of the banquette, every six inches into the front and rear boards. Set a 2-inch wood screw into each hole and use a drill to tighten them completely, screwing the boards together. Repeat the process with another 1/2-by-36-by-24-inch board on the other end to be the other side board.
Put a 1/2-by-24-by-49-inch board against the frame and align the edges of the board with the edges of the frame. Drill holes every 12 inches into this board, which is the bottom board, and through to the front, rear and side boards. Set 2-inch wood screws into each hole and tighten them to attach the bottom board.
Rotate the banquette so the open section of the frame faces upward. Position another 1/2-by-24-by-49-inch board on top of the banquette and adjust the edges so they are even with the frame edges. Slide a piano hinge between the new board, which is the banquette lid, and the frame of the banquette. Mark the location of the piano hinge screw holes, lifting the lid if necessary.
Drill a hole in each marked location. Reposition the piano hinge and screw it in place against the lid and the banquette side using 1/2-inch wood screws.