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Optimal performance of background limited thermal detectors requires adequate control over all relevant sources of incident electromagnetic radiation. In addition to the radiant power incident from the scene of interest, undesired or spurious power can potentially couple to the sensor via its bias and readout circuitry employed to operate the device. One means of limiting the contribution of this stray radiation is to filter or block leakage associated with electrical connections in the detector environment. Here we discuss a fabrication methodology for realizing compact planar filters embedded in the wall of the detector enclosure whose tailored response controls the propagation of light through the far infrared. This approach consists of fabricating an array of boxed-stripline transmission line blocking filters to control thermal radiation incident via this path. Topologically, each superconducting center conductor is encased by a silicon dioxide dielectric insulator and surrounded by a metallic shield to form a single mode transmission line structure. We report on achieved attenuation and return loss and find that it replicates simulated data to a high degree.