Dampers

I have one room directly above my forced air furnace that is extremely warm. I would like to regulate the heating of this room. I have tried closing the register but that does not seem to completely solve the problem. I think that maybe a damper placed in the ductwork directly exiting the furnace would solve the problem .

We live in an old house(circa 1861). The furnace is new about eight years old but the ductwork is much older--rectangular in shape. I know that circular duct work is more conducive to installing a damper.

What would you suggest for this problem and where could I find the supplies to complete this project?

Thanks for your time.