You are going to get custom
sheets for your custom air mattress too? Putting a rectangular mattress on the curved settee would avoid that problem. People are also more used to sleeping on rectangular mattresses. They like having shoulder room, which an oval mattress would put a crimp on. Try getting an Aerobed and see if it fits. You won't be leaving any air mattress out during the day, since you need to sit on the settee, so getting an oval air mattress seems pointless.