Ceiling
Because they're not in contact with the food, glazed tiles are fine. You want a deep dark black tile, sized so that it fills almost the entire oven shelf. You want the gap in the center for the air to flow, but no gaps anywhere else. Use foil to fill in any non center gaps.
If you really want to go the extra mile, you can insulate the ceiling with either perlcrete, rock wool or board (on top of heavy duty foil), but two layers of tiles should help the heat stay in the lower compartment.
Floor
As I've discussed previously, lack of conductivity is ideal in this scenario- no steel, no iron. Quarry tiles are ideal (preferably 1/2" or stacked), if you can find them. Fibrament, although a bit pricey, will work very well, as long as you get the right size (see below). Firebrick, even splits, takes way too long to pre-heat.
With the deflection in place, you might be able to get away with a traditional cordierite baking stone/kiln shelf, if you already have one, but try, if possible, not to go too thick with it. 1/2" to 3/4" should be fine.
Deflector
Suspend the pan, with wire, under the shelf. A shiny pan is preferable, but just about any metal pan should work fine. It should be metal and it should be relatively light gauge so that it isn't sucking up too much heat. The lips on a sheet pan might help with deflection, but they aren't critical. If you need to hack a sheet pan up for proper sizing, that's fine.
Deflection isn't absolutely essential- you might be able to get good results with a quarry tile hearth on it's own. But every little bit helps in taking the heat from below and directing it to the top of the pizza, where you want it.
I can't stress this enough- sizing is critical. The stone has to be smaller than the deflector, which, in turn, has to be smaller than ceiling. At the same time, though, for the best NY style pizza possible, the stone has to be as large as you can get it.
The deflector doesn't need air flow on all 4 sides, so go with a square deflector that's sized to touch the back wall and almost touch the door.
If at all possible, the stone should be 1/4" to 1/2" less than the deflector on all sides. That's ideal. If you have to go with a slightly smaller stone, then try not to go too much smaller. In an 18" deep oven, that means an 18" square deflector and 17" square stone (ideally).
An IR thermometer that goes to at least 700 is critical, as the lower compartment of the oven will be hotter than the top and, without IR readings, you'll have no idea what your stone temp is.
As the pizza bakes, you want the oven dial set to high so that the bottom burner is on.
This kind of scenario can get a little tricky for multiple pies. As time passes, the heat will flow up through the tile ceiling and reach the upper compartment/thermostat, cycling the burner off and potentially dropping the temperatures below. You should be able to get 3 pies done before this becomes noticeable, but more pies than that (or long periods between pies) could get tricky. Not impossible, just tricky. That will be a bridge we'll need to cross later.
Edit: Before you purchase anything, find out the BTU output for your bake burner. Take the depth and the width of the interior (the shelf dimensions), multiply the two to get the square inches. Then divide the total BTUs by the square inches to get BTU/sq inch. A happy place will be between 36 and 55 BTU/sq inch, which, for an 18" x 23" oven, would be between 15K BTUs and 23K BTUs for the burner (with more being better). If you need help with the math, post your specs here.
Let me put this another way... if your bake burner has 23+K BTUs, with a $100 fibrament stone, about $5 worth of tiles and a $10ish sheet pan, you will have the thermodynamic equivalent of an $8000 Marsal MB oven.
Edit 2: I previously recommended either suspending the sheet pan or setting the stone on the pan, with spacers. At these temps, an empty sheet pan will most likely warp, so I nixed the stone on pan idea. Go with the suspension.
