Roger, it all boils down to the style of pizza you're trying to make, which, in turn, is a question of bake time. Fast baked (less than 90 seconds) Neapolitan pizza isn't very viable on a charcoal Weber. You can do 3 minute Neo-NY (some char) or fast NY (puffy/chewy/not very crisp/up to 5 minutes) with the right equipment. For that kind of bake time, this is, to date, the preferred tool for non DIYers:http://www.kettlepizza.com/KettlePizza-Deluxe-KPD-22-p/kpd-22.htm
If you have DIY inclinations, then something very similar can be fashioned out of a strip of sheet metal (no zinc) or flashing.
If, on the other hand, you want an American-ish style golden brown, crispy, with not too much flop pizza baked for longer than 7 minutes, then all you need for that is a stone. As far as choosing a stone goes, that's again, personal taste. I like big pizzas better than smaller ones. A 22.5" weber should be able to accommodate a 19" stone with room for air flow. I just did a quick search for 19" circular kiln shelves, and they tend to a be a bit scarce and on the expensive expensive side:http://shop.clay-planet.com/round-kiln-shelves.aspx
If you're fine with smaller pizzas, this is a quality product:http://www.axner.com/cordieriteshelf-16roundx58.aspx
Fibrament is one last option- and it might work a bit better than a kiln shelf for a 7+ minute bake, but, I tend to prefer cordierite kiln shelves because they're a bit more durable.http://bakingstone.com/shop/barbecue_grill/
One disclaimer: I haven't seen a lot of single stone Weber bakes. The Kettlepizza is fairly proven when it comes to 3+ minute bakes:http://slice.seriouseats.com/archives/2013/06/pizza-lab-kettle-pizza-upgrade-short.html?ref=title
but there's a chance, perhaps with careful fire placement, and very quick turning (to keep the lid closed as much as possible), one could enter a 3-4 minute territory with a Weber and a kiln shelf. As far as I know, though, I haven't seen it.
Also, regarding steel- steel's only possible role in a bottom heat scenario such as a Weber is as a ceiling. If you bake on it, the extreme conductivity of the steel will cause the bottom to burn long before the top finishes baking.