I have made several benches from headboards, but many of them incorporated a matching footboard. Every bench has its own challenges (opportunities?) and usually require design decisions being made on the fly.
The driving specifications are: the finished seat height should 15 to 17 inches from the floor; and the seat depth should be 14 to 17 inches.
Every bench requires its own head scratching, so I will mention a few decisions I made with this one.