Can be made of just 1 pc of 6' of 1"x12" standard wood and 12 screws.
Rigid, minimal, low cost, and comfortable.

Let me share the example of combination of length.
(leg 1, leg 2, seat, backrest)
=(400, 400, 327, rest≈702)
=(400, 400, 365, rest≈664)
=(400, 400, 400, rest≈629)

1. Cut accurate as possible. You can ask the DIY store to cut them.
2. Make sure using narrow screw, or drill small hole before screwing.
3. When joining the seat to two legs, you can use backrest as the guide as in the below drawing.
4. Mark the guide line of the backrest on the both surface of legs. It helps you to fix the backrest to legs.