depends how good you are with your tools. brainstorm, come up with some ideas, walk around the hardware store.. once you have a plan in your head, buy the parts, go home, and put it together. I made my carbon filter out of wood mostly, box shape, but carbon costs a lot to keep it going.
If you look around a bit on the inter web you can easly find a carbon filter cheeper than you can make one for. I payed £40 for 6 x 24 inch brand new.e-bay