Use a replacement part for the Infinity Orbit 245 inflatable kayak, offered by Harmony Gear in the USA. One inflatable inner air chamber, kayak shaped donut, outer side wall, with two air chambers and two Boston valves, 100 inches long.
(Floor chamber is not included.) Might be compatible with some similar brand IKs with inner and outer skins. Other Infinity brand replacement chambers might be available by special order, such as; the Orbit Seat 9800283, Orbit Floor 9800275, or Odyssey parts such as the seats and chambers.