Set a 1/2-by-18-by-18 inch side board with its good side face down on the working surface. Arrange two 1 1/2-by-3-by-1/2-by-17 inch posts parallel to each other and on the side board with their largest surfaces touching the board. Slide the posts to the edges of the side board until their ends are 1/2 inches from each end of the side board, and their sides are even with the sides of the side boards. Drill four holes through the posts and into the side boards and attach them together with 1 3/4-inch wood screws. Repeat for the opposite side board.
Stand the two side boards up on their ends, parallel to each other with the attached posts facing inward. Place a 1/2-by-18-by-48 inch front board perpendicular to the side boards with its ends butting up to the side boards and its side pressing against the posts. Drill three holes through the front board and into each post, then attach the boards together with 1 3/4-inch wood screws. Repeat for the rear board.
Turn the seat upside down and place a 1/2-by-17-by-48 inch bottom board on top of the posts, surrounded by the side and rear boards. Drill down into the bottom board and through to each post and attach the bottom board to the posts with two wood screws in each post.
Place a 2-by-18-by-49 inch foam pad on a 2-by-18-by-49 inch seat cover board. Cover the foam with fabric and stretch it tightly around the seat cover board. Fasten it to the bottom of the board with staples every four inches. Remove excess fabric with scissors.
Center a 2-by-17-by-48 inch top board on the bottom of the seat cover board. Fasten the top board to the seat cover board with eight 3/4-inch wood screws. Set the completed seat cover onto the rest of the seat.