Any reason *not* to use an un-glazed porcelain or ceramic tile from the borg? Only a few $$s?
I have only been using a round, Pampered Chef stone that my wife bought 15+ years ago, so I only know it. I watched Alton Brown suggest that large cheap, unglazed tiles from the hardware store are a good alternative to spending lots of money, but I haven't had my hands one that was larger than 12". If you put 4 of those together, you could have a nice 24" square area for making pizza!
Only the usual negatives of fragility vs stronger materials. But, that's up to the buyer. I went baking steel because 1 I liked the product and properties and 2 my wife WILL BREAK ANYTHING that could possibly be labelled fragile by any stretch of the imagination.:heh:
Hard to beat a baking steel, does everything a stone does better IMO. If you can find a commercially made size/thickness that fits your BGE to your liking go for it.
If you can't quite find the size/shape you want, I'd lookup your local metal fab shop and take a drawing of the size/shape/thickness you want and let them know what it's for and that you'd like the sharp edges smoothed out.
It might not be as expensive as you think to have the perfect baking steel.