The Kayak Shop by Chris K. at CLC has plans for the "tortured plywood" Pokomoke double. This probably isn't the simplest boat to build and it might not be cheap since you would probably want to use a good grade of marine plywood to accomodate the severe curves of the hull. But the cost of the book includes plans and instructions.

Nick Schades "The Strip Built Kayak" also includes plans for a double, but not in plywood. Still, if you have a good source of wood and mill your own strips, you can build a fine boat without spending a lot of money.