Traditional usage has the paddler sitting on a sealskin directly on the frame of the boat. Modern concession is to use a 1/2" foam pad instead of the sealskin. To make this boat, and the next, less tippy, ditch the seat and the 1" pad. If you need something over the frame for comfort, use a thin piece of wood - perhaps 4mm marine plywood. With your seat that high in what is inherently a tender design, your search for stability is asking for magic.

: My keel is about 1 1/2 inches, wood seat is about 1 1/2 inches, and foam
: cusion is about an inch. There is nothing I can do to lower the center of
: gravity on this one, and I know to make it lower on the next. What else
: should I do? How deep should the yak be?