Right now, because I am in a dusty area, I use that stickon material filter stuff called DustX. It sticks well and stretches a lot. I cover EVERYTHING in the back with it. Change it once a month. On the fan, I have it pushing air in and use a DemciFlex 120 filter on it. Even so, there isn't enough air flow to allow the three 140cfm fans to work well at max speed. (run them 7v now).
I'm going to probably fill all the rear holes with steel epoxy or silver-solder some mesh then fill or something. Maybe steel filler based bondo type stuff, anything that will sand well and paint well and not shrink/crack over time. I've used steel wool and liquid steel as filler before too.
Then, I will open up the side window panel area with vents or mesh that I can cover with a custom DemciFlex filter or maybe use a standard triple-120 DemciFlex filter. Something like that. Also, that disk tray in the bottom comes out... maybe the cross piece gets vented more... not sure yet. Also, major repaint job too... I hate all black.
All I know is if I turn the fans on high, the three 120x38mm 140cfm fans in there now are starving for air... If you look at my build logs in sig below, near the end of the LD-29 build I put on the custom filters on the back. Anybody can order anything custom from them and they ship fast and their stuff cleans easy.
So, drill out rivets, sand,cut,hack, silver solder or rivet 4x120 or 4x140 grill on top, repaint sides, vent window side a LOT more, fill all other stupid vents to keep it clean inside... take a while I think.