How to install small damper in a duct?
I would go over sized rather than undersized if you have the room to do it.
An undersized unit will limit your maximum flow, unless you always will be reducing the flow to that area then go undersized.
Strips of ducting can be purchased at most home stores that you can rivet from one side to the other then use a good quality duct tape to seal.
They make a tool to bend the sheet metal makes nice easy bends.
Hope this helps.