Fatman513,
You have been most helpful. Yes a linear diffuser sounds right. I had these ducts made to decrease the sound of the heating/cooling in the theater room and now just want to be able to close off the ducts to decrease the heat in the room etc. The duct is 4 by 48 inches.
I e-mailed hvacquick and see if they have it.
Now that you know what I "mean" if you have a direct supplier that would be great.
Again, you have been very helpful.]
Larry