Is the bench going to be used out in the weather? If you have a jointer you could laminate a panel kinda like the end of a church pew and cut it out. If the bench is used out in the weather it wouldn't do very well even if you used an exterior glue like titebond III. If it was for exterior use I would probably use a 2x10 or 2x12 and cut a curved back leg out of a single piece and make a frame such as a waiting room chair for the bench end.