Stig,
Respectfully, what they are showing is a custom cut/sewn seat cover. I can bend plywood to fit if it is pre shaped.
BUT on the same track... The material they use for vinyl tops on cars is of the same genre, and it really does bend the way you want.
The only draw back is the fact it has no seams, and does look like a pulled over cover.
I guess there are as many ways to cover a seat as there are active posters here. MY first, and only recommend is to find a Marine re-cover shop (I didn't want to go to the trouble of spelling Upholstery), and have them do it.
Auto shops think they are gold, and same for most bike shops. A mom, and pop Marine shop will laugh at how easy this job is, and charge accordingly.
I had to spell Upholstery after all so it didn't save me anything by avoiding that word in the first place. Hell...