When contemplating constructing an outdoor bake oven, the most important thing to ask yourself is what the oven will be used for.
If you are going to just be baking pizza and flat breads, you could construct a sleek beauty with little or no insulation and the oven would have a small appetite for wood consumption.
However, if you have serious thoughts about baking multiply loaf breads with multiple bakes from a single firing, then you want to construct a beast. It will be beastly in that you'll need lots of mass for heat retention and lots of insulation to keep the heat from finding its way to China. The beast will require about 8" of mass and about 8" of insulation, and it will have a beastly appetite to consume wood.
A compromise oven might be one with 4" of mass and 4" of insulation. This oven would work for flat breads and you should be able to get one bake of loaf breads before you needed to refire.
The beast can be a beauty, but the beauty cannot be a beast.