The automotive stuff is really restrictive -- do not want.

Do an image search for "air conditioner foam filter" -- this stuff's really cheap & you can wash it & reuse it.  Most HW stores carry it. Needs a screen/grill over the fan's intake (those old-skool wire ones are cheap & good), and then simply cut a rectangle slightly oversize & tape edges right to the case (or sandwich between two grills if you wanna be racy).

The furnace filters are big (as in a couple of feet), usually disposable HVAC filters that you can cut up and use -- also really cheap but more of a pain.

If you're hosting in a data center, you don't usually want filters (they usually filter their air, but maybe not everywhere).