Previously, we have demonstrated a viable approach based on microstructural and topological modulation of periodically arranged elements to program wave scattering in ferromagnetic glass-coated microwire metacomposites. In order to fully exploit the intrinsic structure of the composite, here, we implement the concept of composites plainification by an in-built vertical interface on randomly dispersed short-cut microwires allowing the adjustment of electromagnetic properties to a larger extent. Such interface was modified through arranging wires with different internal structures in two separated regions and by alternating these regions through wire concentration variations associated with polarization differences across the interface. When the wire concentration was equal in both regions, two well-defined transmission windows with varied amplitude and bandwidth were generated. Wire concentration fluctuations resulted in strong scattering changes ranging from broad passbands to pronounced stopbands, demonstrating the intimate relationship between wire content and space charge variations at the interface. This provides a new method to rationally exploit interfacial effects and microstructural features of microwire metacomposites. Moreover, the advantages of enabling tunable scattering spectra by merely 0.053 vol.% of fillers and simple structure make the proposed plainification strategy instrumental to designing filters with broadband frequency selectivity.