This application relates to an enclosure for a portable electronic device is described. The enclosure can include metal bands included along the enclosure and a support structure. The support structure can include a thermally conductive core that is capable of conducting thermal energy generated by the operational components and rails that are bound between the metal bands and the thermally conductive core, where the rails are characterized as having a rate of thermal conductivity that is less than a rate of thermal conductivity of the thermally conductive core so that the thermal energy generated by the operational component is directed away from the operational component and away from the metal bands.