I moulded this around the clay in a roughly 4 inch thick layer, adding balls of clay about 6" diameter each time like bricks in a wall, then moulding them to shape. Here's what it looked like when it was about 3/4 finished:
The wires you can see on the left side of the last pictures are three thermocouples. all embedded in the dome clay. One is actually inside the oven (in the sand right now), the other 1 inch from the inside surface , and the last one is on the outside of the dome (under the pimple you can see sticking out).
Once I'd finished it a gave it a good hard whack all round with a 4x2 bit of timber to compact the mix, then smoothed it all down with my wet hands. Finally - the oven dome was finished! This was a definite milestone for the project.
The clay dome took about 4h from start to finish to build, and used just under 75kg clay and 75kg grog. The internal diameter of the oven is 32", with 15" height (9.5" door height), and the oven has 4" thick clay walls. Now we have to see how this dries - I'm expecting some cracks, which I'll fill in later if I have to.