Doh! The kettlepizza insert, I was being dense
Bob, the critical aspect of the DIY insert is that the hearth has to be smaller than the ceiling so that the heat can rise up from below and warm the ceiling. If the two stones are the same size, the ceiling is blocked. That's the traditional KettlePizza insert with a charcoal grill perspective.
If you want to go more LBEish/propane burner-y, then you might be able to cut one stone in such a way that the air flows up the back and out the front hole in the 'C'. You'd still need the border of the stone intact, though, for the flashing of the 'C' to sit on. Whatever heat that's coming up needs to a be a relatively airtight seal- with the heat only going out the C opening- not under the flashing.
I think it's easier, though, for both approaches to have a large ceiling stone and a smaller hearth.