Almost all whitewater boats have very large cockpits. But, they also have great thigh support. check them out and sit in them. Your thighs get locked in so you actually have to drop you legs to touch your knees together. This is what you want. You'll have to fabricate them out of foam. Some poeple make a wood piece and add foam to it. Rolling is dependant on your thighs being locked in.

Also, unfortunately, 24 inch wide, hard chine boats are not easy rollers. I learned in a chesapeake one. The instructor, who was fabulous, blew his first roll when he tried my chesapeake 17. I'll never forget that.