A method of fabricating a waveguide filter includes forming a monolithic polymeric dielectric core configured for fabricating a dielectric-filled cavity, where the core includes a plurality of spaced-apart depressions; applying a layer of conducting metal to an outer surface of the core to form a metallized core with a conductive metal layer, wherein the metal layer includes port openings at opposite ends of the metallized core, and wherein metallized depressions thereby formed are the ridges of the filter's ridge-waveguide sections; and mounting the metallized core on a supporting carrier. A desired number of such filters can be connected with an electrical-series-type connection among one port of each filter to form a frequency multiplexer having a waveguide manifold.