Howdy y'all,
Sadly not all of us live in areas that are marijuana friendly so I wanted to show an effective way to build an incredibly simple carbon filter. I know there are probably other carbon filter designs similar to this one, but I just wanted to show my take on it. This design is adaptable to fit any size fan, and in my personal experience it removes a vast majority of even the dankest of smells. This filter is a vertical style filter that is ideal for mounting near the main heat producers of your grow, AKA your lights. With out further adieu lets get started.

What you will need:
Window Screen Material: $6.48

Galvanized Steel Worm Gear Clamp: $1.98

90 Degree Round Adjustable Elbow $5.54

Activated Charcoal $12.99

Total: $26.99

All of these materials are available at your local hardware store, and from our future overlords over at Amazon. I have taken the liberty of trying to link the cheapest materials that I could find for this project. NOTE: I am partial to 6 inch diameter fans, so all of the materials I have linked are for building a 6 inch filter. I also prefer to buy activated charcoal in bulk, but your filter only needs to be changed every 3-4 months. Bulk is always cheaper!

Putting it together:
Step I: Cut a 1'x1' sheet out of the window screen material.
We are using window screen material because I have found that it eliminates sagging due to gravity. This makes sure that all of the air that your fan pulls through the filter will get forced through the activated charcoal.

Step II: Cover one end of the adjustable elbow with the 1'x1' sheet of window screen material.
Note: I went with the adjustable elbow because you have the option to bend the pipe if that fits your grow room better. If you want to shave a couple bucks off of this project, just use a straight 6 inch duct pipe.

Step III: Clamp 1'x1' sheet of window screen material to your pipe using the worm gear clamp. Make sure the window screen material is as fairly taught. Don't worry about ripping the material as it is reasonably durable.

Step IV: Pour the activated charcoal into the open end of the pipe. I find that a charcoal depth of 1.5 inches works very well, as it does not restrict airflow too much.

Step V: Mount and connect to your fan, and enjoy your smell free home!

I have been using an Active Air 400 CFM Inline fan with this filter and it works GREAT! I like to use gorilla tape to tape the filter to my inline fan, but you can also use another worm gear clamp...or really anything that will hold the filter in place.
You will note that my activated charcoal looks a bit different than the one linked above. This is because I ordered about 20 pounds of the stuff 3 years ago for an awesome price. Do not worry they will function the same.

Thank y'all for checking out my first ever post and please feel free to offer any constructive criticism!
PierceStoned