I recently had a new Clydesdale cast iron wood burning insert installed in my fireplace, and the sides fit fine, but the top of the arch of the fireplace is just a few inches too high, so you can see into the fireplace and the top of the insert. I was going to have a steel frame fabricated, paint it to match what's there, and slide it behind the existing frame. I've spec'd it out and it seems doable, but the guy at the retailer suggested I use concrete backer board instead of steel, as it will better match the appearance of the cast iron. Has anyone had any experience with this? I gladly welcome other ideas as well.