The plastic bag geometry should be modeled as a surface, not as a solid. The fluid inside will be modeled as a solid body. After the fluid volume is defined, the plastic bag is simply the faces of the solid body that represents the fluid volume. Those are easily lifted off the solid body by a select, copy and paste operation in SpaceClaim. In Mechanical, the sheet body is meshed with shell elements that are assigned the material property and the thickness of the plastic film.