The photo of your setup reminded me of a piece I read recently at the website theartisan.net about constructing an oven within an oven (a standard home oven). To see the photo of the setup (as used in an electric oven), go to the above-referenced website, click on the link Bread Basics in the frame menu to the left and then the link Oven Humidity & Simulation of a Professional Oven. In the meantime, I have have cut and pasted the text of the article below (in quotes).
"Simulation of a More "Complete" Masonry Oven
Using both a high quality mercury calibration thermometer and a "contact" thermometer of the sort used to measure the surface temperature of a wood stove, Mr. Sole had determined that he can achieve temperatures of about 700° in a standard home oven set to 500°. How is this achieved?
A ring of fire bricks are stood on end on a soapstone baking sheet. An opening is left in the front wide enough for the peel. The oven heats until the oven thermostat tells it that the air within is 500°. It then shuts off the gas, and start to cool. The air cools much more rapidly than the mass of bricks. Assume that the oven thermostat has a "swing" of 50°. When the air in the oven drops to 450°, on comes the gas to heat the oven once again. At that moment, the bricks would be significantly hotter than the air. The gas keeps cycling on and off in this fashion, each time increasing the temperature of the bricks.
At the point when the bricks and air reach the temperatures defined above, the dough goes into the oven. Mr. Sole states, "...The results astounded me. I have used today's recipe for years, but the spring this time was perhaps 50% greater than ever before..."
The Artisan Baker has built and used this simulated masonry chamber in an electric oven, and it works as well in this situation as in Mr. Sole's gas oven. The photo depicts the setup in an electric oven. As can be seen, the weight of the bricks on the rack causes the rack to sag a bit toward the enter. We suggest that prior to using this setup, a call be made to your oven manufacturer to ascertain the estimated weight load that your rack can handle. Lighter refractory brick may be used to obviate this problem, but they are more difficult to obtain. Half thickness brick are available, and would probably work as well if they are not thinner than the spacings on the rack itself. For example, in the oven depicted here, the rack spacings are approximately 1 inch, but the thinner bricks are about 1/16" narrower, and fall through the spacings.
The fire bricks used in the oven on the photo were obtained from Pacific Clay Products, Inc., located in Lake Elsinore, California. They may be contacted via email at Pacific Clay, or visited on the Internet at http://www.pacificclay.com