Assuming I'm envisioning this right, I don't think heating and trying to reform the duct is the best choice. Probably warp and end up with something you don't like.
Two ideas. Either get a file and gradually file the duct until it matches the conture of the bezel - assuming there's enough material on the duct to do so. Other would be to just put it in place and fill in the gap with some high density foam. Maybe even cut a round gasket out of the stuff with the top and bottom slightly thicker. Stuff I mean is pretty stiff, easily cut into shapes, harder to find though. Maybe if you provided some pics others might come up with more inspiring ideas?
