Check for a small handle on either side of the lower vents. Many gas fireplaces have a cover that slides over the vent holes similar to the lower vent holes on a charcoal grill. The handles are the same color as the lower part of the fireplace. If there are handles present, simply slide them to the opposite end of the vent. This closes the vent when the fireplace is not in use.
Measure the length and height of each lower vent. Some gas fireplaces have two while others have three vents. Add 2 inches to the overall length and height.
Cut a section of 1/4-inch-thick magnetic sheet to the size of each vent with a pair of scissors. Magnetic sheets are available at office supply stores and home improvement centers.
Place the magnetic sheets over the gas fireplace vents. The magnetic sheets will adhere to the metal of each vent.