Colbyt, thanks so much for the reply. I was fairly certain cardboard
was a lame fix, based on common sense. There's three more vents to go,
and all three look like they have cardboard liners. Unbelieveable.
Measured up what was here, and bought a few different sizes/styles of
"boots" to make sure I had something that would work. Also got a
couple pieces of plain sheet metal in case I had to make something
from scratch. Also a roll of the higher-priced duct tape, labeled for
hvac repair/maintenance. Hope this will stick longer & better.
I've managed to hack a replacement together. I'm sure a professional
would laugh their ass off, but it fits the wall opening much better
and it's 100% metal all the way to the wall opening. The only way to
improve on what I've done would be to re-do almost that entire run of
ducting, and patch a plaster wall. Beats cardboard and three inches of
open space.There's still some slop, but I've probably reduced the
original amount of gap by 75%. <patting myself on the back>
It was a major pain in the butt, but I learned a lot. The best part is
when the furnace needs replaced, I'll have a basic knowledge of what
ductwork should look like before I make a final payment.