I'm tying to install proper ductwork between a register and a rectangular duct in the basement. The previous owners cut a rough hole into the rectangular trunk and used a piece of eavestrough as the conduit between the trunk and the register. It's only about 2 feet between the trunk and register.
To fix this, I think I need to tap in using a Side Takeoff, run a short round pipe and the end off with a boot against the register. The register is on the side of a finished "conduit box" (terminology?) right under the ceiling. My question is, how do I install that register boot? Do I put it in last, and if so how can I tape/seal the connection between the boot and the round pipe as I wouldn't be able to get my fingers into that space once the boot is in place?