Abstract: A filter frame formed from a stock channel member having a generally U-shaped cross-sectional configuration. The channel member may be provided in stock lengths which may be cut on-site to the desired size to form each side of a filter frame when assembled together. The cut lengths of the channel members may be assembled together in the appropriate configuration to match the filter being replaced, and a mass of filter medium cut to a corresponding configuration and placed within the cavity formed by the u-shaped cross-section of the channel members to form the air filter. The channel members are also provided with a plurality of U-shaped grooves in the upstanding legs of the channel members which form tabs. The tabs may be advantageously employed with a length of elongated material to form a protective screen. Specifically, a length of elongated material, such as string, may be wound about the tabs and across the length of the air filter to be wound through separate tabs.