Improved structures for spatial power-combining devices are disclosed. A spatial power-combining device includes a plurality of amplifier assemblies and each amplifier assembly includes a body structure that supports an input antenna structure, an amplifier, and an output antenna structure. According to embodiments disclosed herein, the body structure comprises a material that is configured to provide the spatial power-combining device with reduced weight while maintaining good thermal dissipation for heat generated by the amplifiers. In certain embodiments, the body structure may comprise an allotrope of carbon such as graphite or graphene, among others. In certain embodiments, the body structure may include one or more thermal vias configured to dissipate heat from the amplifier.