The top's base is a sheet of 18mm compressed cement fibre. I've laid a single course of 50mm pavers on top of this, with the beveled edge down. This will give a smooth surface to the base of the oven.
I'm taking a bit of gamble with just having one layer of pavers, but I reckon it'll be ok. It will just mean less mass for the heat to sink into and release from. I don't think that this will be such a huge issue for a back yard oven.
At this point, all I need to do now to finish of thebase is to split the pavers on the edges to ensure a nice fit inside the steel frame of the base.
The base was finished off on the second weekend of the project. Ready to lay the dome