I made these to fit mat_neu27's Filter System ( see Here
) You can just use the internal one on its own but with the mat_neu27 filter system I used both of them and for me it works great.
I used a cheap thin Carbon Cooker hood filter to line the inside of the tubes - cut to 135mm then rolled and tacked the edges together with Super Glue ( Gluing the carbon filter causes a small smoke / reaction so use small dabs and do this in a well ventilated area) - then push it in and trim the end flush with the top. AIRO Active Carbon pellets ( any active carbon should work fine ) If you are using both filters with the added EE filter system like mine I used the crappy sponge supplied with the printer - cut in half / shaped to fit tight in the bottom of the tube to hold the Carbon Pellets in place. Happy Printing