Hmm, then that sounds like there will have to be way too much heat below before there's enough heat above. Pizza needs substantial heat from above, and I can't imagine how that set-up would get enough heat above it without being far too hot underneath. I've tried jerry-rigged versions of this before, and always had the problem you're having. But the box you're using is sold commercially as supposedly working fine over a gas grill, right? So then I'm mystified why it would work over a gas grill. Have you looked at reviews of the item and seen if a significant number of people are actually using it successfully for pizza over a bottom-heat source?
Works at a residential alternative high school in the Himalayas SECMOL.org . "Back home" is Cape Cod, E Coast USA.
I use a DIY version of this on my gas grill.
Mine has firebricks sitting on the heat shields directly.
That puts them inches below the grill surface.
The pizza goes on the grill surface in a perforated steel pans.
To cook the top of the pizza, I place bricks in a three sided "U" shape, on the grill surface.
The bricks are laid on their flat side.
On top of this I place a tray filled with more firebrick.
With this in place, I light the grill and crank it all the way up.
After 15 minutes the "pizza oven" is ready.
The thermal mass is less than an inch from the top of the pizza, but 2 or 3 inches from the bottom of the pizza.
In this way the thermal mass redistributes and evens out the heat.
Bakes great pizza, perfectly even top and bottom, but half my family dislike the "grilled" taste, the cretins!
Based on all of that, I think adding a huge chunk of thermal mass between the rocket exhaust and the bottom of the pizza will slow, spread and store the heat.
If there isn't a chunk of steel or ceramic in the roof of the oven, it might not be enough.
Rebecca, I don't see any reports of my issue with this type of oven over a gas grill. So, how can a 5" batch box be hotter than 3 burners on a gas grill?
William, I'd like to see your setup. The original Bakerstone pizza box has stone on the floor, roof, and sides. There's only a thin gap at the rear which lets the heat in but I presume that the floor of the box is also heated by gas.
I'm going to try a commercial pizza dough to see if that is the issue. I can't see why changing from butane to wood gas makes such a difference. More likely it's a mea culpa with the dough.
Location: Ladakh, Indian Himalayas at 10,500 feet, zone 5