I think someone should develop a 2 module design for this, where the Stirling engine is separated from the fission reactor with insulated pipes. Why? Weight savings. Insulated pipes can be made very efficient and relatively light. Radiation shielding can't easily be made light. By separating the modules into two sections, martian soil can be more easily used to provide the radiation shielding. It could even be built out of bricks, which we already know can be made out of martian soil.
That might be a good second phase setup for a larger base, but this current one would be good for initial setup since it could be ready without having to produce anything on planet. That makes it good for rovers or other unmanned setups.
