bag o' beans
I used a couple of them for years on my boats. They're great. Even undeway, heeling. Toss it into the leeward corner of the cockpit, next to the pulpit and steer with one foot on the wheel. With someone to keep an eye out to weather, it's Nirvana! Remove about a third of the styrofoam pellets to make them more form-fitting. They're cheap, easy to use, easy to store (toss them into a corner) and easy to replace if needed. I bought two of those folding seats now, but a bean bag has its place.