I had a pile of salvaged 2x12 boards, 4" long. I drew a couple sketches & made some simple, but sturdy benches out of them. The seat and backrest could be made out of other sizes of lumber & you could change some of the other lengths too -- whatever works and doesn't fall over!
The cut diagram and the photos should be pretty self-explanatory. In some of the photos (e.g. showing the braces under the seat), the bench is upside down. My construction is pretty loosey goosey -- these will go on beaches where a little unevennness is no big deal.
These benches are heavy! Not especially pretty, but you could saw a nice curve on the "armrests", paint them or add a cushion.