nicozeg,
Here is a link to the Thomas Registry search for "Thermal Conductive Materials"
This document has a line of thermally and electrically conductive adheasives/paints.
Looks like the 805 one part system might be your best bet... Dielectric strength of 50 volts/mill and thermal conductivity of 12.5 Btu-in/hr-ft²-F°.
There are probably better thermal conductors with a higher dielectric strength... but this was just a quick search.
The idea I had was that you could spray/coat the FET area with something like this... then build up your walls around the coated area. Leaving a single material for the heat to go through as silicone is generally a thermal insulator.
Someone correct me if I'm wrong... ( It HAS happened before...
)