Here are some photos of my double decker pizza oven, the result of about 4 years of trial and error.
1. First take my 1990 OT and cut a slot in it. This is the way it worked for some time with a regular pizza stone raised up on bricks, and I was always happy with this. But once you seek perfection, you cannot stop.
2. Bought 50cm and 40cm round kiln shelves. Cut a slot out the former for my pizza stone, and suspended the later for a lowered roof. For a while I thought this was perfection, until the stone in the lid cracked.
3. Tried a variety of other solutions in the roof, including stainless steel, hence all the bolts in the lid now. But I couldn't get the results I was getting previously.
4. Bought the Onlyfire Pizza Oven Conversion Kit. Definitely needs a roof to stop the heat escaping, so instead of using the handles provided threaded some long stainless steel rods to rest a stone or whatever on top. Bought some 5mm stainless steel sheet, cut to size with a jigsaw (need good blades) and then used the handles supplied on that so I have both a pizza oven roof and a Plancha. Well, perfection achieved again.
5. What to do with the old lid? I decided to cut some stainless steel to fill the gap, so I could use it as a regular BBQ.
Today something amazing happened. I was cooking pizza and I decided to experiment with a pizza stone on top of the roof, and the slot left on the lid on top. A two slot - double decker arrangement. Air temperatures in the bottom reached 750F and in the top 550F. Wow. Using two paddles, we would place one in the top with a fresh pizza for about 30 seconds, this would make it much easier to throw on to the stone on the bottom for about two mins.
I think I've found my pizza nirvana.