An aftermarket damper must have a lockout to the furnace incase it
doesnt open, There have been alot of cases where they failed leading to
Co issues. I think the cost will be excessive compared to a new unit
with a starting high efficiency, meaning payback is poor. You could be
reducing furnace efficiency and harm the furnace if closing vents
increases temp rise to much. A friend bough a house where the ac coil
was clogged shut, he removed it to clean it and his bills dropped in
half. You really need your system serviced to find out if it is running
optimaly and go through your options for an upgrade.