[MOfj40]

The blockoff plate would work fine. You can also purchase in-duct damper kits, allowing you to install a butterfly style damper into existing duct. You just need to get the duct dimensions before you order.

I'd tell you to check the gas pressure at the regulator, but it sounds like you probably don't have a manometer. It might be worth a service call to a reputable hvac company to have them check/set the regulator (abou $50-75 depending on area). If the gas pressure is significantly off it can cause a high bill due to the furnace running rich.