After an hour or so of mixing, we moved over to the base of the oven (which Matt had made earlier). This was constructed from four pallets nailed together, forming a box. This box was then filled with demolition rubble. You can use anything you want though – earth, rocks from the river, car parts. Anything that will keep it steady.
The top of the box is lined with straw and glass bottles. This to help insulate the base of the oven, to stop heat leaking out and downwards into the base. The base itself would be made from a layer of common-or-garden house bricks, then a layer of special, heat retaining fire-bricks. We used bricks salvaged from storage heaters for this. However, apparently you can also use block paving bricks (from suburban driveways, start keeping an eye out now for looting purposes).