This one has been on my to do list for way too long, and I finally found the method that works the best for me. For a bit of a run down on the process, for those that wanted more information.
I started with the cheap porn ball (or gym ball), and made a cardboard opening, then coated that with cement and newspaper.Once the first layer hardened, I added another thick layer of vermiculite and cement, and let that set./Colum Black-Byron
Then taking that off the ball, I moved it into position on top of the fire bricks, raised up the back by around 30cm, and put the a thick layer of vermiculite/cement around the base, and over the whole mold.
Then more concrete/vermiculite, until it got to around 15cm thick, and a decorative coat of darker concrete. I let it set for a week, fired it up with a small fire first, then after another day put a decent fire in there, and started making pizza!/Colum Black-Byron