The 100W bulb did a good job in seeing the frost off, so I made the mould for the clay dome. I made a central box out of bricks with my lightbulb in, then surrounded the whole lot with masonry sand. The bulb in the bricks should keep the sand warm and stop it (and the clay) from freezing.
This took 3 bags of builders sand (25kg each) but I'll scoop all this out later and reuse it for mortar. I could really have done with more sand, but I used all my broken bricks in the middle of the pile to bulk it out a bit. This mould will give me ~32 inches inside diameter of the oven.