This paper proposes a novel technique for the design of miniaturized Ka-band filters based on locally resonant metamaterials (LRMs), which are fabricated monolithically using lost wax casting. We implement ultra-small metamaterial filters by exploiting a novel subwavelength guiding mechanism in an evanescent rectangular waveguide, which is loaded by ultra-small pins, forming a composite pin-pipe waveguide (CPPW). To guarantee compatibility of CPPWs with existing WR28 interfaces, we propose a unique subwavelength method, called meta-port, to significantly improve the matching. Finally, we fabricate and test three ultra-compact WR28 filters made of pure copper in the frequency range of 26-34GHz. Our measurements demonstrate the customizability of the bandwidth, low insertion loss, and high rejection level for 6th and 7th order filters. These filters, whose total length are less than the operating wavelength, are significantly lighter than traditional filters. They may find applications in future satellite systems and 5G infrastructures.