Position a 1/2-by-18-by-20-inch board, which will serve as the side of the bench, on the working surface and set two 1 1/2-by-3 1/2-by-19-inch boards, which will be the interior supports, on top of it with their large flat surfaces touching the side board. Make the interior supports parallel to each other and near the edge of the side board, so that each interior support board is 1/2 inch away from the edge of the side board on three of its sides.
Drill four holes through the support board and into the side board, being careful not to puncture the other surface of the side board. Attach the boards together using 2-inch wood screws. Repeat to make another side board assembly.
Stand both side boards upright with the help of an assistant and slide a 1/2-by-20-by-52-inch board, which will serve as the front board, between them and perpendicular to them so that the front board is touching both internal support boards on its side, and its ends are pressing against both side boards. Make sure that the boards are pressed tight against each other and are in the shape of a U.
Drill four evenly spaced pilot holes through the front board and into each interior support board. Attach the boards together using 2 1/2-inch wood screws placed in the pilot holes. Repeat the process for the rear board.
Turn the bench upside down and set a 1/2-by-17-by-52-inch board, which will serve as the base of the bench, onto the interior support boards. Drill two holes down through the base board and into each interior support board. Attach the boards together using 2 1/2-inch wood screws.
Set another 1/2-by-17-by-52-inch board, which will serve as the seat board, flat on the work surface. Place a 2-by-17-by-52-inch layer of foam on top of it and drape the entire seat with fabric. Flip the seat over, holding the fabric tightly to the foam and board, and begin to staple the fabric to the seat while the fabric is stretched tightly. Place staples every 4 inches, tightening the fabric each time so that the fabric is smooth. Place the staples 2 inches from the edge of the seat board. Set the completed seat on top of the interior support boards.