If you like building sand castles, you'll love this part! We're building a sand dome that forms the shape of the inside of the oven. After the adobe dries a little, we'll dig out the sand and viola! there's a hollow little cave for fire and food.
We want the cooking area of the oven to be 14" tall, so we cut a strip of wood that long and stuck it in a small heap of slightly moist sand on top of the firebricks, then started building the dome to the height of the stick.
Keep the sand as dry as possible but moist enough to be molded into the dome shape. The base circumference of dome should be the biggest circle that fits within the confines of the firebrick layer.
Now cover the sand dome with a layer of moistened newspaper. Later, when it's time to hollow out the over, the newspaper layer will help you know when you've removed all the sand -- at which point you'll stop digging so you don't weaken the still-drying adobe walls.