A:Answer Hello,
Thanks for your question! You would need to start with the 6 inch round as the duct attaches to the motor collar itself then they can transition to a rectangular, but unfortunately you can't start with rectangular due to the duct attaching directly to the motor. You can find more detailed information on our website https://zephyronline.com/product/tornado-i-range-hood-insert/. Thank you for your inquiry! - Zephyr