I'm installing a 4 inch fresh air flexible duct. Would like to install an airtight damper that will allow me to close the intake air from entering the stove when not in use. I don't want to pay the $170 for the ones the stove retailers advertise. Has anyone found a simple low cost solution? For example, just use a 4 inch pvc ball/gate valve, or some other ingenious DIY solution? Thanks!