Does this image show what you are looking for? The dewar_solid is one material and internal to the cryostat that is a separate material? From a meshing point of view, I have to have distinct boundary on the cryostat that is shared by the boundary of the dewar_solid. The solids must be manifold. I produced this mesh with the following commands and then turning on the clipping plane so we could see the internal tetrahedral mesh and structure.
reset import step "C:/Users/karl/Downloads/Cryostat_assy_simplified (1)/Cryostat_assy_simplified.stp" heal remove overlap volume 1 8 modify larger separate body 8 delete Volume 62 imprint volume 1 63 merge volume 1 63 vol all scheme tetmesh mesh vol 1 63 draw tet all