Wow, that is a lot of cushion. When I built boats, the upholstery dept used a material called Starboard for the seat substrates, I believe. Here is a link to a company that has a similar product.
The material used in my 94 PS 205 seems a little more dense than the Starboard and may hold a screw a little better plus you could probably get by with a little thinner sheet, but either way it is still an extruded plastic of some type. You may need to have some kind of support leg in the middle, as that is a pretty long span.