So I took a trip to the hardware store (was actually there to buy something to kill off the pine scale afflicting some of my pine bushes) and picked up a pleated furnace air filter for 4 bucks. Something like this:
It seems a little too thick for computer case fans to really produce good airflow through, but I thought I'd give it a try on the large side panel of my case (my front panel has a pseudo-dust filter already to my recent discovery).
The panel in question:
The first step was getting the pleated filter out of the cardboard. This wasn't too terribly hard, though it was a lot more time consuming than I expected, as the filter material is actually spot glued to the cardboard in numerous places. The glue wasn't too terribly troublesome as the glue spots were small, but there were a lot of them. I had to take care not to bend the chickenwire and tear up the filter material. Otherwise, this was pretty easy.
Here's the filter freed from its cardboard prison:
The next step was the most time consuming and required the most patience. I had to flatten out the portion of the filter I intended to use. I had to resort to a combination of a heavy book, rolling pin, and good old elbow grease. I probably spent half an hour wrestling with the material trying to get it as flat as possible. After I achieved the flatness I was looking for, I measured, traced, and cut out the desired piece. The dimensions for the mesh panel including a little overlap on each edge were 300mm by 223mm (yes, mm, I am a man of SCIENCE).
Here's the result of measuring twice and cutting once (and a heck of a lot of flattening and straightening):
It was a perfect fit on the first try:
After marking and cutting holes for the fan I was able to mount the fan directly onto the filter material without needing washers to prevent the fan blades from touching the mesh, likely due to my diligence in making this as flat as I could.
Here's a shoddy picture of the end result:
I haven't bothered sealing around the edges yet as I'm not sure if I'll keep this dust filter. I'm going to keep my eye out for something better that has a little more air flow but is similarly rigid and durable for easy removal and cleaning. I would have used some of those scrub pads but I can't find any that are large enough for the size of this mesh panel without having to sew, tape, or glue them side-by-side.
Feeling with my hand, I can already tell there is less airflow moving through the side panel and also out the top of my case, but it does still seem to be maintaining positive pressure. I'll watch my case temps and see if they seem elevated much, though I don't have a pre-filter baseline for comparison.