Ok, first of all I'm fully aware heat rises but I say natural convection be damned I've got fans! This was intended to be a ridiculously cheap and easy mod (if you can really even call it that) and honestly I'm not hugely concerned that it made my case/CPU temps rise a couple degrees (literally ~2ºC). ...feel free to Dremel the crap out of your case and install the intake on the bottom if you must, I'm happy as is. Anyway...
The pictures pretty much tell the whole story but I'll ramble anyway... I wanted to cut down on dust in my case and after some thought this seemed like the best course, I set all my fans to exhaust so the "only" (edit: I am aware of cracks, etc) air coming into the case is from the 140mm vent on top of which is sitting a $12 canister vacuum filter, that's it. I dig the aesthetic, it fits in with the Bio-Wheels on my aquarium filters, and without any actual modding it was a perfect fit first try, really "clean" (pun intended). I figure it'll be easy enough to clean the filter as it collects dust, no big deal if it isn't sitting up there for a day or so while it dries...
The only thing remaining is going to wait until miss Sandy B. is able to move into her new home, like many I'm waiting for a B3 mobo but when I get it the whole system gets a good scrubbing and this bad boy gets installed for air pressure reasons (I mean, I haven't just given up on those 2º ). I've never been able to fit a vent fan in my current build because the CPU socket is so close to the top of the board, doesn't look like that will be the case with the P8P67 I'm waiting for...
Bit of advice if you want to give this a try, let the filter air out in your garage for a week or two after you take the shrink wrap off it and before you slap it on your PC, it's a little smelly, those plasticizers needs some time to breathe out... Case is an Antec 300 BTW, YMMV with other cases of course.