You can design the bench around a cushion or have one custom made. Call a local fabric store or design studio and find out if outdoor fabric is available. Cut the pieces according to your specific design.
Attach 2" x 6" front seat supports 12 inches up from the ground to the outsides of the front 4" x 4" posts using 3-inch stainless steel screws. Repeat this step for the back seat support to the back 4" x 4" posts.
Stand up the front posts and the front seat support assembly. Then stand up the back posts and back seat support assembly about three feet apart to create the depth for the bench.
Attach the 2" x 6" x 3' sides of the seat to the outsides of 4x4 posts. The whole assembly should be connected together at this point. Four corner 4" x 4" posts standing upright and connected together by 2" x 6" seat supports 12 inches from the ground.
Begin attaching the two 2" x 4" seat slat supports to the insides of the front and back seat support pieces. They should be recessed 1-1/2 inches on the inside of the seat supports so that when the seat slats are set in, the seat slates are even with the tops of the seat support pieces. Attach the pieces with several stainless steel screws. These pieces support the cushion. Lay in 2" x 2" seat slats every six inches and screw into seat salt support pieces with 2" stainless steel screws.
There are two options in securing the top crosspieces to the 4" x 4" posts.
Option 1: Cut the notch in the crosspieces and cut out a mortise or a hole in the posts. Insert the notch into the mortise using glue, screws and a clamp.
Option 2: Attach the cross pieces to the posts with a screw to either the inside or outside of the 4x4 posts.
Using a semitransparent penetrating stain, brush a thin layer of stain on bench and let it dry.