Both my top 120.3 rad and side 120.2 rad suck air in from the outside. I made a rough and ready filter by putting a pair of tights over the rad grill. I didn't want the material to be on both sides of the grill though, as that would just trap dust between, and look a bit daft.
So what I did was stretch the material one one side of the grill so that it was taut, just use pegs to keep it in place. Then ran a line of cement glue around the back , about half a cm in from the edges so it is not visible. Let it dry and cut away the material from one side. What you are left with is a single sheet of stretched filter one one side of the grill.
Mount it with the filter side down such that it is between the grill and the case. When clean you can hardly see it, and obviously it shows when it is collecting dust. I just brush the dust off periodically. Make sure you do this with the fans off, otherwise they can catch if you push the material down while brushing.
Keeps my rad fins and case internals virtually dust free.