get a 120mm bi metal hole saw and make some new fan holes
We used to do this often back in the day - making "blowhole" (top) fan locations. Blowhole fans are very effective at increasing cooling efficiency but if your case sits on or near the floor, the top mounted fan may produce some undesirable fan noise. And I hate fan noise!
One word of caution - take everything out of the case before drilling your 6 holes (one pilot hole, one big hole for the air vent plus 4 mounting screw holes) and before edge grinding/finishing. Then blast out (or even carefully hose out) the interior to ensure no metal filings fall onto or into any of the electronics, or are left behind. A pain, but worth it.
You will need a
grill too and I recommend getting a quality
diamond lapidary wheel to finish the edges to save your knuckles from the jagged edge caused by the saw. Blood is hard to clean up. Last, covering the cut edge with
edge trim gives it a professional appearance and offers more protection for your knuckles from the sharp edge of the sheet metal. Note that some local hardware stores sell this edge trim by the foot. You might check some auto parts stores too for car door edging/molding - but I have found case sheet metal to be too thin for the automotive variety.
Be sure to position the fan hole far enough back so the mounted fan does not interfere with an optical drive - but not so far back that it interferes with a top mounted power supply. You may have to opt for a 92mm or 80mm fan if not enough room so before to measure before buying the hole saw. But note smaller fans tend to make more noise as they must spin faster to move the same amount of air as larger fans.
I really hate fan noise!
Since blowhole fans typically exhaust heated air out, you won't need an air filter. But if you decide to cut the hole in the side panel for an intake fan, a
dust filer might be a good idea. But note side panel fans sometimes disrupt the desired flow of air through the case and in some situations, actually degrade cooling - unless they blow directly into a tube and channel the air directly onto the CPU or GPU. But also understand side panel fans cause more vibration in the large panels - thus may make more noise. Did I mention I hate fan noise?
If all this seems like a lot of money, it is. So I would not invest that sort of money unless adding fan holes to cases is something you will do more than once. If this is likely the one and only time, then I would just go for a new, quality and quiet case instead.
