Clean your fireplace thoroughly and paint it, if necessary. This will make it easier to install the fireplace reflector properly.
Measure the back of your fireplace. Most commercially-made fireplace reflectors come in the following common sizes: 15 inches by 15 inches, 16 inches by 18 inches and 20 inches by 20 inches.
Buy a sheet of heavy duty, 14-gauge 304 stainless steel in the appropriate size to fit your fireplace. It should be at least as wide as the area occupied by the fire or gas flame but, if your fireplace is large, extend the width by a few inches to maximize heat reflection. A standard large screen is 20 inches in width.
Construct a fireback support by placing four to six standard fireplace bricks in pairs along the back wall floor of your fireplace. Stand them on their sides, parallel to the back wall. Place the steel sheet between the pairs of bricks so it stands vertical to the floor of the fireplace, held in place by the bricks.
Adjust the reflector so it points directly toward the room, not at an angle. Also adjust the bend of your rim so it doesn't interfere with the draw of smoke through the flue.