Bob, Bob--you've got to get that "steel" mentality out of your mind. This is saltwater (for most of us), so we need to use aluminum. It is also lighter than steel--big plus. Weight, as we know, is everything on these boats, especially if we want to use them for camping (not everyone does.) In the construction of my "expedition" hakas, I used pine boards, marine varnish, aluminum framing, stainless steel screws. (
http://www.hobiecat.com/forums/viewtopic.php?f=70&t=7276&start=240, scroll down to "Construction of Expedition Hakas.")
If people try to use plywood, you get into a catch-22 situation. The plywood is heavy, and marine or not, may not stand up to the toughness that hakas require. If you try to stabilize the plywood with varnish or fiberglass, that is just adding to the weight. If you use thin (1/4 to 1/3" plywood,) you have to reinforce it with metal bracing, driving the weight up--and, how do you attach the metal bracing to the thin plywood.
If you are not worried about weight, fiber-glassed, metal-reinforced plywood might work, but there are many other easy (heavy) options for hakas on this thread.
BTW, Bob, I'm don't know about the white PE (polyethylene) cutting boards at Walmart, but those boards can be heavy. REI sells very light-weight PE cutting boards for camping. Getting truly light-weight, strong hakas (say, 10-11# each) is a tough puzzle.
Keith