Bucket seats not all plastic
The bucket seats in three of my Wenonahs are kevlar, the last is fiberglass. Molded yes, cheap no.
What works on my web seat is a…
…folding plastic backrest with front edges which hook on the front of the seat (hard to describe). Had it for years and can’t name to source. I also attach a section of a surplus Army sleeping pad (closed foam) with Velcro straps, about 3/4 inch-luxurious!