Asbestos panels fit into the slots.
Unfortunately I have discovered that I can set the epoxy alight after a few seconds using a blowtorch, and that is no good. There is likely to be arcing and bits of molten copper flying around nearby.
What else could I make the insulator from? I doubt that Formica would catch alight very quickly so I could machine it from that. Or maybe concrete reinforced with fibreglass. Or pottery. Any other ideas?