How can I fix this furnace filter slot to prevent filters bending?
Assuming you can find a channel at one of the big box stores you can slide the channel in and maybe screw it in all the way at the end so the screws don't interfere with the filter. In fact it seems like you don't even need a piece of channel. A piece bent like an L should work. When you screw it in offset it so the top of the L is not above the top of the filter. Attached is a drawing.