So far I've got these leftovers:

• A piece of aluminium, 2 mm x 120 mm x 120 mm,
• Carbon/aramid fabric, 188 g/mm², plain weave,
• Two-component silicone,
• Braiding from some old cable for connecting individual carbon filaments.

All of the above can handle temperatures of at least 300 °C (limited by the silicone). Aramid will loose its strength at high temperatures, but the design will not rely on that parameter. The first things to do now:

• Get a bag of expanded glass spheres. I'll use these with silicone to create a spreadable, flexible and thermally insulating filler;
• Measure the resistance of a single carbon filament and figure out how I want to/have to connect them to create a decent heating resistor.
• Sand that piece of aluminium until it's sufficiently flat;
• Create fixtures to prepare the top surface (aluminium with a silicone border) and the fabric. I'll have to pull out all unwanted carbon filaments that would create shortcuts, without messing up the overall structure. This step will be close to a nightmare so the fixture will have to facilitate this step.