having done the same thing in a boat years ago...
I used 1/2" plywood because the seats had to cover openings to storage below and I wanted to fasten other stuff to the underside of the seats
I did not use batting and I probably would not use it if I was doing this again...I would worry about the stuff moving around/bunching up with every day use.
I also used 5" of foam...very comfy...could probably get by with 4".
IMO getting an electric stapler is worth it's weight
in gold in making the project a success. An electric stapler does not cost much and makes the job a breeze...tightly burying the staples in the fabric...a manual stapler with it's recoil will drive you nuts...again IMO
If I was to do it again I would use a few dabs of glue between the plywood and the foam to prevent "foam creep" with/during every day use
all in all the project was a great success....my seats were held in place by way of cutting the plywood exactly to some retaining features in the boat as well as cleats underside (I did not see any retention measures in the project in the link and I think this could become a problem during travel)
great solution for sure and certainly cost effective