I have a 2 story house built around 1910 that was changed to a duplex sometime in the 1960's or 1950's. The furnace is a unit that was put it in the 1970's and uses new(er) flat and round ducts. However some of the returns are not even ducts. They put a flat metal sheet across 2 joists and stapled it on and ran that back to the furnace. They are the old ones with a floor grate. Could I just take the metal off and basically make a false duct (basically install a duct in the floor and run it in the joist for a foot and let it open into the basement with a mouse proof cover) a for a foot or so in the basement and allow the furnace to pull its air from the basement? Or do I need to duct all the way back to it? (remember its just the return)

I hate these things because every single winter the mice get into these returns and I have to clean them out, plus they look ugly...