I use fans for every things in my cab: extraction, intraction, ventilation, but i dont use any ducting. PC fan dont give enough pressure to extract through a duct longer than 1-2ft.

I made my own carbon filters. I have one attached to one 12cm fan extracting, but the filter low largely the flow from the fan. I needed to put an aditional fan extracting (i use 220w of CFLs).

But you can made a carbon filter and put it out the cab, with a fan. As larger the carbon surface and thickness, larger the odor absortion. And as slower the air passes throught the carbon, the larger odor absortion. It wont work as well as a carbon filter in the extraction, but reduces smell a lot.