• Brushed a bit of fresh silicone on the (partially cured) top plate
• Poured some of the silicone into the frame and put in the plate/resistors
• Connected the supply wires
• Filled all gaps with silicone/expanded glass bubble filler
• Added thermocouples below the carbon: one close to the center, one close to the clamps

The result:

Measured resistance: 0.7 Ohms (series), so with a 12V supply it should dissipate about 200W.

Then I sandwiched the two parts and now I can't wait to see the result.