This is a basement intall. The fresh air intake ductwork is 3" hard round galvanized and is behind the drywall (with faux rock veneer). There is approx. 4 inch stub-out of exposed 3" duct prodruding out of the wall, which is provided for attaching the stove's 2" semi rigid aluminum fresh air intake hose. How the heck do I attach 2" aluminum "flex" duct to the 3" rigid duct? My extensive search finds no 3" to 2" duct reducing fitting available on the market. Any ideas greatly appreciated.