I've read about wood frame,covered with painted fabric. Sounds simple to do, inexpensive too. Very lightweight also. Different degrees of sophistication, from sawn frames and paint drop canvass, to kevlar roving and aircraft dacron covering.
Popular Mechanix had a few such designs way, way back, 50s maybe, that may be available on microfich from the library. Lot's of modern info for that type to be had. Probably the easiest type to build, IMHO.
I read about Nessmuk's canoe, marvelous piece of work, but rather serious construction methods, steam bent ribs, copper clinch nails, and so forth. I believe he christened it "the Sairy Gamp", after a Dickens character who "never took water". Light weight plywood designs are available that will fit right in with your size requirements, and would be perhaps easier to build than cedar planking.
May I offer a thought about river rocks? Ya'll be careful now, ya' hear?
Since you own a couple of boats, you're experienced enough to know your size requirements, and that boats "shrink" when you put 'em in the water.
It was two years ago, this month, that I was returning home from a long drive, on a rainy windy afternoon. As I was going along, a car turned in behind me, and the two boats strapped to the roof caught my notice, wooden kayaks, uncovered, and in the rain. Wooden boats are made, not sold, so it caught my eye, and I figured he was taking them someplace to store for the winter. After all, no one would go out boating in such weather, at this time of year. I wanted to talk to this person, and find out about his boats, and thought that if he turned off, maybe I might have a chance to stop and ask a few questions.
He just kept right on going, nice and steady, behind me. Now, ahead, several miles was a lake, that I had to pass on my way home. But, naw, no way, not in this weather! So as the miles went, the suspense built! Sure enough, he turned down the lane to the launch area. I durn near killed myself making a u turn!
So, we met, he and his wife, and yes, he made the 2 kayaks from kits, and yes, they were indeed going out, they wanted to see what it was like in this kind of weather. The wind blew the hat off my head, and the lady was shivering from the cold wind, and had to go into her car and don her wet suit. I got my questions answered, and watched them go off. I've met them since, so they made it back.
I gained a new respect for the capabilities of small boats.
Woodbutcher