Heat exchanger assemblies for electronic devices are disclosed. A heat exchanger assembly may include a heat transfer body that has a face that forms open passageways. A cover structure may be attached to the heat transfer body in a manner to enclose the open passageways, thereby forming a heat exchanger assembly that includes enclosed fluid conduits. In this regard, the enclosed fluid conduits may form complex and intricate patterns within the heat exchanger assembly that are tailored to the heat requirements of a particular application. Heat exchanger assemblies as described herein may be thermally coupled to a center waveguide section of a spatial power-combining device. The enclosed fluid conduits may be tailored based on locations of amplifiers within the center waveguide section to provide improved thermal operation of the spatial power-combining device.