nealtw
Contractor retired
So in the picture we can see the end of an electrical cable so some more work might be done with that. In the same picture we can see some very dirty insulation. That says some dirty air from the house has been leaking there and that wants to be stopped. You might find it easier to fix from upstairs or even working thru the duct from above and that way you might find ready made parts that you can make work.
I would think after you have something started down you want to fill all the gaps around the new duct work with a fire rated foam to stop all air leaks from the house to the attic.
Then you have other heat registers that should be tighten up too.
I know all this sound like ugly work and it probably is but warm moist air leaking into attic is food for mold it will certainly destroy a house but can be deadly for people too. And there will never be a better time to fix it than when you have things already under way.
I would think after you have something started down you want to fill all the gaps around the new duct work with a fire rated foam to stop all air leaks from the house to the attic.
Then you have other heat registers that should be tighten up too.
I know all this sound like ugly work and it probably is but warm moist air leaking into attic is food for mold it will certainly destroy a house but can be deadly for people too. And there will never be a better time to fix it than when you have things already under way.