I will get some stick for this but....
Use some ordinary WBP (exterior grade) ply. A whole sheet will cost less than a 1/4 sheet of the marine stuff. Hopefully your seat base will not be permanently submerged or subject to excessive force, so you do not need the extra quality.
Make sure you seal it well. Several coats of paint, varnish or resin on both sides and most importantly the edges.
"A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools" Douglas Adams