Unfortunately there are not a lot of choices for a round ceiling diffuser. I did a search and found this not commercial looking and should work with a 6 inch vent.

Here is a link that might be useful: Ceiling Diffuser Antique Grille

mike_home Thanks so much for finding this, I'm going to check with the contractors today to make sure it will work. Hopefully it does because it looks a lot better than the rest of the ones that are available.

There are many decorative grilles available with a rectangular shape, but I don't know if there are any available which will designed to work with a 6 inch round duct.

You could ask your contractor to make a custom rectangular termination at the ceiling. This could be relatively easy or extremely difficult depending what is being done to the ceiling.

Maybe?

Here is a link that might be useful: Possibles....

Thanks saltidawg, they look very nice! I'll call them tomorrow.