This is the larger of the two capital D food-grade hollow silicone gaskets. It sits on the lower flat part of the interlocking lip.
We have them custom extruded out of a special high-tech, high-temp resistant food-grade silicone.
Use black Permatex Auto silicone gasket maker to attach, installation instructions in the private KK forum owner's maintenance area.
Use this to attach the gasket: Permatex 27037 Optimum Black Gasket Maker 3.35 oz