I use a ~35x40cm, 8mm thick, ~8kg slab of iron as a baking steel. It does take a while to heat up, and conversely also to cool down again, but it produces extremely consistent and good results.

As someone who's cracked several conventional baking stones, I would be seriously surprised if the Slab O' Steel doesn't outlast me. I can highly recommend getting one, it's definitely worth it.