I bought a "Frost King Polyurethane foam air conditioner filter" at Home Depot and just cut it up to make filters. Pretty low restriction, and there's a lot of it,1/4" thick, 15"x24". Part#F1524, bar code 0 77578 03051 7.
Problem with computers and filters is that the enclosures are not built to accept filters with a large surface area, so you need extensive mods to get that, or accept some airflow restriction and frequent cleaning.