As you can see, it's a wider stone that's raised to the widest part of the oven.
You can support it from below or build some sort of shelf for it. You can also insulate below it for better efficiency, but that might be necessary. The trickiest part is getting a stone that will fit. Just like the bottom stone is flush against the sides of the wall, the stone positioned in the middle can't have large gaps for wood to fall through.
That higher position position will be night and day for top heat, though.