I have never messed with sheet metal duct work so I am just taking a shot in the dark here but I wonder if a company that installs duct work for heating and air conditioning might be able to fabricate and install a new sheet metal insert for you. Or perhaps you could measure and have them fabricate it and you could install it your self. I would think in this case that it would require more than one piece in which case you would want to make sure that where the 2 join, that the top part has a small fold such that it goes inside of the bottom part. Hope that made sence.
BTW What are the inside dememtions of this shoot?
