Scott, yes, order was shipped about 20 minutes ago. I figure for the price of a couple good dinners out the deal's paid for. I'm thinking of a ceiling drop to just above the opening of the insert with a gap in the back of the bottom stone for flame/heat to come up, hit the dropped ceiling and come over the pizza and a gap in the front of the dropped ceiling stone plate so when the heat comes to the front it will be sucked up thru the dome with the top vent open to bring it in the back again. I certainly will post pics of the new toy and it's results. Thought of doing the DIY and thought what the hell, work an extra shift at the hospital....done, overtime is ALWAYS around ;-)
From the work that Kenji and others have done, I don't think you want any gap in the dropped ceiling. I've scoured the web for hours looking for round stones larger than 21" and I couldn't find any. Hopefully the gap between the KettlePizza insert and Axner's 21" round shelf will be negligible. If it isn't, filling it, without using foil, could be tricky. I guess, in theory, the area closer to the door should be a bit cooler than the area at the back where the flame is coming up, so perhaps, if you push the ceiling back, as you're planning, foil might work in the front to block that gap. Ideally, though, there should be no gap. All the air should be venting via the KettlePizza opening.
Keep in mind that Axner can get a bit creative when it comes to shipping and handling charges, so what looks like an amazing deal may not end up one by the time all charges are added up. It doesn't make that much of a difference, though- the final axner price is competitive with other online ceramic suppliers, AND, more importantly, you WILL want to use this setup with wood, and the Fibrament grill stone cannot be used with wood, so the price comparison between Axner and Fibrament is moot. Do not buy a fibrament stone for this grill.
Also, as Brian pointed out, you're getting a stone with the kettlepizza- a very durable cordierite stone. The hearth is take care of. As I'm sure you're aware, you're looking for a ceiling. Unless you can find a good price on a 21" round kiln shelf locally, I'd go with Axner.