Since basically the only thing in a carbon filter is activated carbon could you not just buy a pipe and fill it with activated carbon from lets say a pool store ~$6/lb USD and save some dough, anyone done it?
yup and then hook a fan up to it to pull air through and maybe get some small air filters for each end of the pipe to hold the carbon in
Not sure how the people around here feel about linking to other forums but a quick search and I found this. http://www.gardenscure.com/420/construction/120203-diy-6in-carbon-scrubber-cbn.html All the parts can easily be found at any hardware store. I have built about 5 off of a similar design. Once you understand the concept and are a little handy you can build it to fit your space. A small room and you could do the inner tube with 4" duct and the outer with 6" as well as make it shorter. Good luck
By just putting active carbon in a pipe will not do the job properly. The fan needs to pull/push air through a large area of (around 1in thick) active carbon. If the carbon is packed much more than an inch thick an extractor fan will have a hard time pulling/pushing air through the carbon.