I am finishing my basement. One of the areas will be a family room. There
are 6 inch circular ducts running between some joists. I shall install a
drop ceiling using the Grid Max/Ceiling Max system. This will give me a
tiled ceiling about 2 inches below the joists. I shall need heating /air
conditioning in the family room.

Are there any registers available which I can attach to a circular duct with
a maximum protruding distance from the duct of 3 inches? I have seen small
circular registers which adjust by rotation of a central knob but I don't
know where. Can anyone help?

Peter.