The cordierite stone I use right now was purchased from a ceramics kiln furniture supplier. It's a 1/2" thick round and it has handled direct flame (actually buffered with fire brick in the little black egg, but the flames do hit the stone directly as well as evidenced by the distinct flame marks on the bottom of it.)
I've spilled room temp water on it when it was 500 degrees hot, baked high hydration bread on it at least 15 times and made close to 50 neapolitan super high heat pizzas with it (lost two pies with sauce and ingredients on it, making a total hot, wet mess) but still--the cordierite holds up!
I would love to have a custom stone for my oven, as I think I know exactly what to expect from cordierite.
It can handle all my screw-ups! ha.
I wonder if the 14 x 16 Old Stone Oven brand I can already purchase would be the right size for my oven (taking into consideration the added circulation of air since it won't be a snug fit: is that a good thing or a bad thing?), or if I should take you up on the offer to make me one. I just started researching larger stones today when I ran across this thread.
I am all for supporting the "mom and pop franchise", and would much prefer to buy from an individual such as yourself, if the cost isn't prohibitive. If we can come to a reasonable agreement I may be interested in testing out your composite. I'll send you an internal message here.