I need to install a bypass type whole home humidifier. It's an Aprilaire bypass type. If my duct work was rectangular cross section metal it would be no problem. My furnace has a small plenum section that is covered by circular transition fittings that go immediately to circular flexible duct. Same on the suction. I cannot find a source of adapter Tees that would allow me to tap into the circular suction duct and there is not sufficient space left on the metal plenum to mount the humidifier. I have thought of building an external box to mount the humidifier on and connecting the box to the plenum or one of the discharge ducts. Has anyone installed a bypass humidifier on a furnace with virtually no duct sheet metal available, only flexible circular duct?