I messed a few things up on this first try but it doesn't show, only if you look closely, hehe ^^. I don't have a ton of sewing machine experience, the stitching isn't always straight.
The zipper plaque was a bit too narrow, getting the backing board in was super tough (will make zipper longer on other back rests).
I'll document what I had trouble with when I have a bit more time. As always, I'll put it up on our wiki.
While removing the cushion I also found Katakana writing on the wooden backing. The writing is hard to see (pencil on wood).
I can make out: バック レスト(Bakku resuto, "back rest")
Our boat was manufactured in Japan(Yamaha), and we keep finding Japanese writing in the strangests of places.