I am looking to patch a large hole that is arround 6" square in the bottom of a steel 8" by 12" duct. The duct had and actually still has a humidifier that hangs down from the duct from 4 finger bolts and has a large hole. I have never figured out how to reseal the duct which is why the humidifer is still on even though it has long since stopped working and is no longer connected to the water either. I don't think they make such humidifiers anymore or I would consider replacing it.

It is 40 year old heavy stainless steel and I don't really see any joints other than where it joins the furnace several feet down otherwise I'd simply replace the section.

What should I do?

Go to any home store. Get a piece of metal for the duct (it's sold in 2' x 2' and similar size pieces). Also pick up some 1/2" hex head self taping duct screws (the kind with a point that is also a small drill bit). Finally, you'll need some silver foil tape. If you don't have one, get a hex driver to fit the screws.

Remove the humidifier, cut the metal so that it overlaps the hole by 1" to 2" on all sides. Attach the metal with the screws and tape all the seams with the foil tape.

You're done.

Thanks it worked like a charm.