Do it yourself Smoke filter

[sillyrib]

So I've been buying and reading up on smoke filters for the past year or so (because of odors and resale value of my house)and as I see it all they are is a fan running air through a carbon filter.(or water for the Venta) Most of us here are not concerned so much with hepa filtration of particles so much as the smoke stinking up the house. Hepa filters are expensive. Carbon filters are not. The people selling these filters mark up the price of the media when in reality, the media(carbon) is cheap.
So I was thinking all you need is a box fan and some bulk carbon filters that you can buy at home depot. Put the carbon filter on the front of the fan and tape it down. Voila, instant powerful air purifier for $20.00 or so.
So where to get bulk carbon(coconut shell) filters?
I would also retrofit the existing filters I have with cutouts from the bulk sheets.
Anyone else have a setup like this? I thinkits ridiculous to pay high prices for the filter and media when you can build the same thing for a fraction of the cost.