This is the type of collar I use when connecting to an existing duct. They are simple to use. Cut out the correct size hole, then bend over every other tab. Put the collar up to the duct. The unbent tabs will fit into the hole. Reach through the collar and bend them over. It holds nice and tight. I have always used duct tape, but they say to use silicone to keep the collar sealed in tight. So, that is what I use now.
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