You're assuming a certain temperature for the surface and mediating heat loss by surface properties - emmisivity (e). Getting e less than 0.01 is rather difficult

I'm adding a protective thermal barrier so that the surface temperature no longer needs to match the internal temperature. This is how most organisms and planets regulate their temperature.

It becomes trivially easy to maintain temperature if a thick enough insulator is available.

For instance the Earth maintains an internal temperature of 3,000 K despite a tiny heat production amounting to 65 milliWatts / square metre, because it has hundreds of km of rock as the insulator. At the surface, it still radiates to space (or more precisely, is in time-averaged equilibrium with sunlight and space) with a surface temperature of ~290 K.

But this has become tiresome, so let's go on to other things...