Hello.
I have an older home with a Rheem furnace. It has a slide in filter which fits under the unit - there's a 2.25 x 17" panel that can be removed, and it looks as if the filter slides in there.

It also looks as if the unit had a cooling coil or heat pump added at some point. (It's an aluminum finned upside down V looking thing). When I purchased the home, there was no filter in the unit, so I crawled up under the furnace box area, and the coil was covered with a quarter inch of felt looking dust!

I cleaned the fins and dirt off as best as possible, but now I cannot find a filter for it, or figure out how the filter stays in place? I don't know if a custom fit filter would just slide in - seems it would fall in below the furnace.

If someone has an idea of what would work, I'd be grateful. Thank you so much.