The 3 inch (75mm) Sparks Overland Diesel Heater Tent Adapter lets you easily zip your heat duct into any zippered door or window! Let’s face it the round duct was never intended to be used with a zipper! We solved this problem with a unique design allowing the slots in the adapter to catch your zipper in slots for a tight fit! No more unwanted cold spots or having your zipper separate and open!