From what I can tell, the largest round-ish stone they have is a 21" 12 sided shelf, which, imo, is probably worse than the 21" round axner shelf, because of the gaps from the 12 sides.
I did notice that all these places have 12 x 24 stones. If you wanted to go the Kenji route and put the stone on the top of the insert, you could put two 12 x 24 stones together.http://www.axner.com/cordierite-shelf-12x24x1rect.aspx
It would, again, be too high, imo, but it would be pretty simple to do, avoid aluminum, create no gaps and give you a Kenji pie- you could do a lot worse than a Kenji pie.
No matter how you shake it out, Axner's 21" round stone in the KettlePizza insert is going to be complicated. There's not going to be any supports in place above the door for the stone to sit on, so you'll most likely need to drill holes in the insert. If you want something that's just ready to go, without any DIY fuss, I think two 12 x 24 shelves is the way to go.