For swing seat, material relates to design and vice versa. You can make long rectangle with or without backrest. Normal swing seat can occupy two for adult and four for children. Too big seat is not recommended, especially indoor installation. You can try round model as similar to half sphere. It takes effort to make because you need proper wood and technical information. Another design is side-by-side sitting. It is for two persons sit and their back lean each other. You still need partition but not too high as half of human upper body. Make sure stand for this swing seat is strong.