Build a 4" tall round slab of adobe on top of the stone base; this round slab will serve as the foundation for the oven itself. The diameter of the stone base is 48", so the foundation slab has a diameter of 48" too.
Next, build a ring of adobe on top of the slab. The diameter of the ring (measured from its outside edge) is 40", which means there's a 4" lip of the foundation exposed all the way around the ring. The walls of the ring should be 4" wide and 4" tall.
Fill the ring with empty wine or beer bottles from your recycling bin. The bottles should be laid on their sides, with the necks in the center and the bottom of the bottles against the circumference of the ring. We found it best to use 1.5 liter wine bottles; each is about 4" tall when laid on its side, so it fills the 4" tall ring quite efficiently.
The air pockets form an energy barrier -- this insulating layer prevents the stone base from stealing heat from the oven.
Finally, it's time to cap the insulating area with -- you guessed it -- another layer of adobe, this one about 2" thick, so the insulating layer is about 6" tall.
Next, add the firebricks like this: