The Dual Coolant Led Lithium (DCLL) blanket is one of the concepts being investigated as candidate for DEMO, due to the high thermal efficiency provided by the flowing PbLi self-cooled breeder at approximate to 700 degrees C in the high temperature design. Key elements are the Flow Channel Inserts (FCIs) serving as electrical and thermal insulators to mitigate MHD effects and to keep the He-cooled steel walls below its maximum allowable temperature due to corrosion. A material based on sandwiching porous SiC between dense SiC layers is proposed for FCIs. In this work results of theoretical calculations and an FEM model are presented to determine the optimum thickness of both porous core and outer dense layers to assure the required thermal insulation across the FCI with minimum thermal stresses, considering achievable properties for the porous SiC material and its fabrication possibilities. It is concluded that the porous core thickness must be at least 5 mm if a porous SiC with thermal conductivity around 7 W/mK is used; a dense coating of approximate to 200 mu m is considered as optimum regarding the thermal stresses present in the FCI.