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To stop cold air leaks from coming in your unused heat register vents just cut up one of those large magnetic calendars for your fridge. I got 3 covers from a Dollar Tree calendar. You can paint them if you choose.
Magnetic Sheets is a great way to cover a heating/air conditioning vents! We rent an older home, and most of the heating/AC vents are rusted wide open. Since our landlord isn't interested in fixing our oven, we know she's not gonna repair the small stuff, either. Most of the time, the vents are fine as-is, but the one under my sewing table either roasts or freezes my feet, depending on the season.
Using a flap from a cardboard box, cut it to fit over your furnace register. On one side of the flap, place two strips of magnetic craft tape. Place the flap onto the register with the tape side down. If properly sized, it should work nicely. I would suggest that you place the tape on the longest sides of the flap.