aluminum, or glass, dont bother with wood.. even pressure treated will rot over time and adds lots of weight to the boat most of all when water logged after rains.
A mix of Aluminum frame with NIDACORE H8PP deck is light weight, but the materials are gonna run you 300-500 but will last a life time.. you could do a alum frame using a/b or a/c exterior grade ply encased in epoxy and epoxy mat... cost in materials 300-400 .. of course the labor will be an additional 200 - 400 for most who know what they are doing .. or you could do it your self with pressure treated frame and a epoxy encased exterior grade ply wood floor..
With this kind of stuff you have to weigh your options on the cost.. and what you want in a extension, do you just want extended deck, do you want to add more usable storage as well .. if so add compartments will add cost pre made hatches are'nt cheap and custom fab wont be any better ..
If its done correctly a well made deck ext and carpet with storage is gonna run you between 500 - 1500 also decide if you want it removable or permanently attached.. food for thought
