My bypass humidifier is designed to run in a loop between the supply and return, and has a damper to close airflow for the summer. The problem is that the damper is by no means airtight when closed and in fact makes the furnace significantly louder when the damper by whistling when it's closed. I assume it hurts the air conditioner's efficiency to keep it open, or else the damper wouldn't be part of the humidifier.
How can I fix this? It looks like gaskets are produced for rectangular blade dampers, but not for round dampers. Is there some kind of plug I can use like a giant cork, or some other way to seal the damper?