Just an FYI:
I just watched this video. IMHO, this is a really good tutorial that involves both scenarios that I described:
- you have access to a ‘free’ and and
- you do not have access to a free end
They use duct sleeve and duct wrap, and show you how to install both.
They also show slowing down a furnace fan speed. This has some risk as a DIY project, because it’s important to keep the temperature rise within the manufacturer’s specified range. Altering the fan speed will quite likely affect this temp rise.
Good luck!