I have seen numerous older furnaces and now more and more newer installs with the following filter set up. Filter rack on the old ones, media cabinet on the new ones screwed to the side of furnace where the side is cut at roughly 14x23 but the filter size is 20x25. I see this as being a waste of some filter. Why not just use a 16x25 ? Also with a lot of these the return drop is 8x24 straight down to an end cap and most times that drop needs to be bigger.