Plasmonic filters based on subwavelength nanohole arrays are an attractive solution for creating arrays of filters with varying passbands in a single lithography step. In this work, we have developed a fabrication method which allows fabrication of nanohole arrays in silver by use of a thin layer of aluminum oxide, which serves the dual purpose of both capping layer and hardmask for metal patterning. We demonstrate arrays of gold and silver mid-infrared plasmonic filters, fabricated on silicon, intended for use in optical filter blocks or for future integration with infrared imagers. The filter arrays are designed for the wavelength range 2-7 µm, and exhibit peak filter transmission efficiencies around 70%.