Before I got my wood stove I burned in an open fireplace for nearly 15 years. I made a grate out of steel the first year and about 5 years it had warped and burned out. At the time Dad and I were doing alot of work for a local scrap yard so I went looking through their piles on ewekend. What I wound up using was part of a drain grate like you see on the side of the road. It's a really fine grade cast and is dished in the middle shich keeps the wood from rolling or sliding off. I cut mine to shape and made a set of bolt on legs for the rear and a decorative piece for the front that combined legs with a raised section to keep logs from falling forward. I burned with it for nearly 10 years and when I took it out to install the insert it looked as good as the day I built it.
Unfortunately finding a grate like that in a scrap yard nowdays would be all but impossible due to so many being stolen and sold when scrap prices were so high. But, if you can find one it will last long after anything else you can get.