Get yourself a bunch of cardboard and some good tape (cardboard and masking don't get along. I recommend Scotch ) and make up a template for your transition. When your happy with that, copy it to some galvanized sheet metal and you screw it together like stovepipe. Some Duck tape to seal it and you should be good to go. You can see from your duct work how they make their connections. Be careful and wear cloves, that stuff cuts like a chainsaw.