Check out www.nativewatercraft.com There is a canadian canoe/kayak hybrid company that has quite the following as well, but I can't remember the name. You could also build a greenland yak with traditional techniques and make the opening as large as you want. Of cource you could to that with Yost's designs as well. Maybe combine the two concepts, cross sections with the middle couple without the tops, but strengthening the gunwalls with a curved deck beam. If you want to make one yourself, your are going to have to be creative as no one will put out plans for a performance oriented boat with a very large cockpit.
Scott