How do I add a filter to the return air vent of my retrofitted forced air system?
Of course, shortly after posting this question, a little more searching yielded the answer, as detailed in this video.
The vent has been cut and prepared for the installation of a filter rack, but one has not been installed yet.
A U-channel needs bent and installed to hold the filter. After installing the U-channel, it needs snap locked and riveted in place. A cover can be slid into the s locks on the edges of the hole.