[rtkwe]

Depending on the conductivity of the material being used could you not have it sink it's own heat through conduction into a pad connected to a beefier version of the ISS ammonia to radiator cycle or combine it with a phase change cooler that melts wax into liquid then slowly radiates that heat away? Even in an atmosphere you have to deal with all that heat eventually.

Truthfully I don't think the best way would even be to try to 3D print the whole structure though just generic pieces that are then assembled into larger assemblies. Just being able to mass manufacture extruded profiles and sheet metal would allow you to do a lot in space that would be hard to launch piecemeal from the ground. Then smaller specialized pieces to join them together in the specific way required for the current vehicle being built could be printed and assembled.

There's a lot of neat research going on in assembling structures like that atm.