Some very clear photos of earth oven construction. Note the rounded base, made with blocks readily available at Big Box or small suppliers.
This link should show part 1 as well. Note the lava rocks used for insulating the base. In Guatemala we used something we called "puma," but I think that's how you say mountain lion in Spanish. Anyway, the material was volcanic in origin.
Here's a little bit of commentary from the person who built the oven. Breadnerd.
A rounded base made with unmortared bricks. www.eriding.net/