Your question was clear enough, and my response described how you accomplish that with a SU model.
SU doesn’t really have true solids–it’s a surface modeler that does not create internal “solid masses.” But a model using the conventions I described, namely a group or component recognized as a “solid” by Entity Info with the faces oriented so that the face backs point toward the interior of the “solid mass” will be interpreted by a 3D printer as you describe, and the space between the walls will print as filled with solid material.
-Gully