Implantable medical devices include an enclosure that is constructed by machining of a material rather than by forming or stamping. The machining produces one or more internal features within the enclosure. These internal features may include shelves that may act as a stiffener and create separate compartments within the enclosure. These internal features may include contoured edges along the shelves to accommodate conductors and other structures that extend from one compartment to another. These features may include slots that are present in one or more locations, such as on a surface of one of the shelves. These internal features may also include standoffs that establish a gap between an internal component and the external wall of the enclosure. These internal features may also include different thicknesses in different areas of the enclosure, such as one wall thickness in one compartment and a different wall thickness in another compartment.