This might sound like a simple question...I have to connect 4" heating duct (galvanized metal) between two existing duct openings due to rearrangement in my basement. How do you get the piece to fit? It appears like you need to start at the supply end and it only installs all they way to the end. But I need to piece it out, but I cannot get the section in because it is essentially too long because it needs to fit into both existing ducts? Do I need to use an external collar on one end? I essentially have two crimped ends on one section, and the section is longer than the opening? What is the trick?