Have I mentioned how handy Dave is?
We tried to use the best looking rocks to build the stone ring. As the ring grew taller, we filled it up with crappier rocks and broken concrete. There's no mortar here -- filling the ring as we went along resulted in a very stable base. We also continually used a level (an app on Dave's iPhone, actually) to make sure we weren't building the Leaning Tower of Pizza (ha-ha, get it?).
Lest you think I was standing idly by while Dave toiled, please know that I also carefully selected rocks and placed them into cozy niches to form the ring. Every time I placed a stone as I saw fit, Dave would look at it thoughtfully, and then quietly move it somewhere else. I knew that the clay would be my metier, so I gave in gracefully, but I warned him that I was going to be the one in charge of molding the clay shape of the oven itself, and that just might turn out to be a sparkly unicorn with wings.