The “coal in a flowerpot”-idea really didn’t work at all, so here’s what what I came up with instead last weekend:

I tried it with a 3-cm-vermiculite board from the hardware store and some heat-proof cement. Here’s the outcome – it still has to cure a little (according to the package) and I’m curious how it’ll work out. (See the other parts of the process if you like: Part I, Part II, Part III, Part IV)