Cardboard to line furnace vents?

Discovered one vent (so far) where the "box" inside the wall was simple cardboard. The furnace is probably 12 years old, ductwork is probably older than that. The duct itselfs comes up about 2" short of actually contacting the cardboard, meaning there are gaps where I can see directly into the basement.
Was this some sort of normal hack years ago, or some nimrod's idea of a cheap shortcut? It certainly seems dangerous to use paper products around a forced-air gas furnace.
Is it feasible for an average handyperson to construct a new "box" with duct board or sheet metal & mastic? I have tin snips & other basic tools, although no way to make decent bends if the sheet metal is too thick. Duct board is new to me. I've worked with drywall and greenboard, however.
Thanks.
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This had to be a nimrod. No HVAC person or even a self respecting handy-person would do it that way. Air in the ducts never gets more than 125 degrees so I don't think you have a fire hazard on your hands.
The boxes as you call them are normally sold premade. They are called register boots. If you are coming off of a round pipe, you may be able to find what you need at the borg. If you are coming off other duct work you may need to buy a couple of pieces to make it work.
No matter how abnormal it is tin, snips screws and patience will allow you to connect it. You might need a hand break or seam bender to bend some of the metal.
Colbyt
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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.
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