Here's some detail on my outdoor oven, by request!
So, let's see...I was in a hurry, and wanted to use as little refractory as possible for this oven. The idea at the time being that I wanted to be able to pick it up and set it on top of a barrel easily. I still think that a relatively thin refractory oven "shell" is a good idea. You can build the mass out of free cob, and just use the refractory to eliminate the possibility of sand in your pizza.
So, like I said, I was in a hurry, and in my normal style I just glued some sticks together in my shop and made a little stick igloo. Covered it in newspaper to fill any gaps and started slapping refractory on it. I started with dense refractory, 130lb/cu.ft, and used about half a bag maybe? I think I got it about an inch thick or so, then switched to lighter refractory, 60lb/cu.ft and I do believe I mixed in fiberglass strands into this layer. I doubt my two layers were a good idea. It isn't separating, but if I wasn't imagining myself carrying the thing around I would rather have gone all dense and then insulated from outside. Anyway, here's some pics...