Open Fireplace–Firebacks.

Steel firebackRandom notes on Firebacks …

Steel is better than cast iron, as it is better at reflecting heat into the room.

Cast iron is better than Steel, as Cast Iron Firebacks will retain the heat better, which means it will continue providing some heat even after the fire has gone out.

You can make your own steel fireback by following these instructions.
Or you could buy a ready made steel fireback from Amazon.

Cast Iron.  Wagga Iron Foundry makes new Fire Backs.  If antique is your thing, firebacks.net can help.