sunshinessmile: the method i posted above will work. you cut two notches on the left hand side of the cartridge, one near the top and one near the bottom. You do the same for the other side of the cartridge.
Place the carbon in the center few spots on the cartridge and rubber hand it together with two rubber bands. as long as you don't shake it violently no carbon will fall out. i was doing this for several months before stopped using carbon.
i can take some pictures if you want more details.
-chris