That's interesting, although my reason for using adobe is simply that it is readilly available, and especially if you network around with local building companies, you will be able to get hold of plenty they are digging out of the ground where they intend on laying foundations, that is destined for the slag heap. So, free, and does provide a good working oven, certainly enough for the average household, and provided it is protected from the rain these have been known to last for a hundred years (apparently!).
From what I have seen, a clay/sand/water first layer, covered after by another which has straw or better still sawdust incorporated into it helps insulate the oven so it loses less heat.
I am curious, where do you get your terracotta from? Can you post a photo of a finished oven made from it, I would love to see one...