If it is a Lennox/Honeywell unit you can buy a 16x25x5 filter at HDepot and slide it in. There are 3 and 4 inch versions too but you may have to buy them at a heating company as they are rare. I have a 3" version that fits in skinny Trion units. You can put them in other brands but they can be loose and sloppy and air passes by them and dirt if they are not a tight fit.
"Cut it twice and it is still too short".