Well, I would install what Lennox recommend - a 1" filter. Having said that, I wouldn't use the 1" pleated Filtrete filters as they produce a high resistance to air flow. Use a cheap fuberglass filter. If you want very small particle filtering, then get a 4" filter box installed and use a honeywell MERV8 or MERV11 pleated filter. You can get higher MERV filters but I probably wouldn't go higher than 11.

If you can get a 2nd one in there, and you switch to those cheap-ass see-through filters, then use 2.

do a pleated 1" under MERV 10 and changeout every 2-3 mths...

IMO

I don't recommend using two filters. It will restrict the air flow.

Why not use a 2 inch filter? Here is a 16 X 25 X 2 Merv 8 that may fit.

good idea by Mike...sometimes these filter racks can be problematic if the correct size is not used...while the size Mike lists is the nominal size, it is not the actual size...see his link...

IMO

I’d love to use a 2 inch filter, but there are rails (probably 2 centimeters) that stick out on the inside of the housing. Those rails are what the pictured filter is resting on.

Then just use the 1" pleated model and changeout every 2-3 mths...

IMO

You mentioned this is a new unit. What did the installer tell you to use for the filter? If they didn't tell you what filter to use, then why not just call the HVAC contractor and ask them what size of filter to use?

There is also a 16 x 24 X 2 filter which has an actual width of 23 3/8 inches. That may fit. If not then use a good 1 inch filter.

This debate has gone on here forever,between the same people and the same people keep giving the same advice. Take your pick,there's bound to be one you like. Here's my 2 cents based on decades of service and thousands of units. So that you can use logic and your own observations,here's what I want you to do(I've did it dozens of times over the years while training service techs and building supers). . So we get this right,in your picture,if air flow's from bottom to top when blower is on you may have filter upside down. There's an arrow on filter showing air flow direction. Put a piece of crap el cheapo filter like recommended above in the slot air passes through first then put a Merv 8 filter in slot that air passes last. You will find that when Merv 8 filter is dirty,fiberglass will still look like it has lots of life left before getting dirty. What that tell's you is anything smaller than a leaf or feather isn't captured by fiberglass and wind's up in the Merv 8. Had that Merv 8 not been there all that lint and dirt would stick to the wet evaporator coil. In a few months enough stuff will collect on coil to restrict air flow through coil. Cooling performance will get worse as more stuff collects on coil and you call a serviceman to add some"Freezon". If the tech was trained by Klem or one of Klem's understudys he will clean the coil and unit will cool better. Note I said cool better as in better than before tech cleaned it. The unit will have suffered some degree of damage that can never be reversed so it will never again cool as well as when new. If the tech subscribe's to cheap filters he will fumble around,say you need to replace unit,give you a bill and go on his way. So here's the deal Stately. Try this and make your on conclusion rather than letting Mr I Heard or Mr They Said make decisions for you,both those guys are known to be big liars. Mr Heard and Mr Said's aurgument is that good filters restrict air flow. Well duh,what does all that crap do when it sticks on coils do beside's restrict air flow? After the test, start using 2" filter from then on. If 2" will not go in slot,see if center piece that devides slot is removable. BTW,you can repay us for our help by coming back to tell us what you observed and what you think about el cheapo fiberglass filters after the test.