I built and paddled one at sea last summer. Used Dralon in lieu of duck canvas, and I did not have to waterproof or latex the hinge. From one ten-foot panel.
Sprayskirt recommended. Broaches.
Some plans (those from troop 303 I guess) show it with a recessed coaming. That may be a good choice for a 12 footer.
I believe an additional folding skeg would stop the twirl (yaw?) movement on every paddlestroke.
Otherwise, here are two links:

http://foldingkayaks.org/build.html