Position two 2-by-12-by-16-inch boards parallel to each other on their short, thin ends. Set them about 2 feet apart. Suspend a 3/4-by-12-by-36-inch plywood board across them so it hangs evenly past the edges of each of the smaller boards, which are the bench couch legs.
Drill four holes through the plywood board, which is the seat support board, and into each bench couch leg. Attach the boards together by drilling 3-inch wood screws tightly into each hole.
Set a solid 1-by-14-by-38-inch board upside down on the work surface. Set the assembled seat support board and legs upside down on top of it so the seat support board is flush with this large seat board. Drill eight holes through the seat support board and into the seat board. Set 1-inch wood screws in each hole and tighten them securely to completely attach the seat board to the support board and legs.
Flip the bench couch back upright and set a 1-by-14-by-38-inch piece of foam on top of the seat board. Stretch fabric across the top and underneath the seat support board. Staple it in place on the seat support board, spacing the staples every 3 inches. Continue to stretch the fabric tightly and staple it in place until it is completely attached. Cut the excess fabric off with scissors.