My furnace is leaking air. The fitting that connects the hot air supply elbow to the furnace is too smal. There is about 1/2-1/4 gap. What is the most realible way to fix the gap, without replacing the dutc. Looking for long term solution. I don't think alumin tape would last long.
Note: The pic show similar issue. The ducts are only 4 years old (new home).