A waveguide filter with a signal input port at a first end and a signal output port at a second end includes a dielectric core of moldable material where the outer surface of its periphery has a metal layer with nonmetallized openings positioned at opposite ends of the filter to accommodate the input and output ports. The filter's periphery is configured to provide a cascade connection of a plurality of metal-bounded ridge-waveguide sections with interspersed metal-bounded evanescent-mode coupling regions. The filter can be joined through a manifold to realize a frequency-multiplexer, with the manifold containing a cascade connection of electrically short waveguide segments and quasi-lumped waveguide circuit components, such as irises. The filter and multiplexer are amenable to the application of cost-effective injection molding techniques to manufacture the dielectric core.