This paper trial did well under pressure and weight, evenly distributed, I think I might have to make up a cardboard one to better test its ability to take weight in the main seating area (the middle and to the back).

I also think that this concept makes the inserting of the back rest very difficult, and it may use up too much of the material.

The curvature around the middle and towards the front helps to stop the structure from swaying back to front.