That’s much clearer now understand the requirements better. Would a slab of stone - marble, quartz or granite - work for that? Could be 400x400x38mm. We can add a pair of handles to it. I’m thinking of offcuts used in kitchen worktops (free). That will withstand the heat and should be flat enough too. It also has surface already finished and smooth and just needs cutting to size, smoothing edges and drilling holes for fixing the handles. But maybe not so great as a heat sink. Could perhaps have sheet metal on that if needed - a copper plate perhaps?
@palmada - how about a Makerspace special edition brew with custom beer bottles! Check out the cool stuff that Dorine can do.