From my father in law...

It is called a smoke guard. Mine has adjustable extensions so it fits most fireplaces around my size. Most fireplace dealers would know because it is a common problem. But if you go for that option, bring a photo and measurements. It may be the case that the metal frame front you have will make it difficult for an adjustable guard like mine to be really effective against escaping smoke. The ends of mine press against the side wall of the fireplace and the top presses against the underside of the bricks forming the inside top. If that seal is not tight you could still get smoke in the house. There may be other ways to seal it.