As you accumulate more properties, you end up with central AC units of all sizes and ages.

Many of these require odd size filters you can't buy at the big box stores, so what do you do?  Do you:

(A) Mail order dozens of custom size disposable filters?

(B) Mail order custom size permanent washable filters?

(C) Buy those adjustable plastic grids from the big box stores and cut them to size and insert those green filter into them?  The adjustable plastic ones don't look like they would last too long...

(D) Buy one size smaller and tape it to a wood stick or whatever to make up the difference...

(E) Find a way to build a wood or foam box around the inlet and install a "common size" grille...

(F) Go "COMMANDO", don't use anything LOL - which is essentially the same thing if you make your tenant be responsible for that.