I have tried freebase in the GVG, and the vapor is very nice.  However when I go for extended sessions, I tend to get sore lungs that feel dry.  Granted, not as sore if I were to smoke instead and DMT isn't the only thing I vape..

The little glass tube in the picture is a glass screen to hold the carbon pellets in.

I guess I should have made this clearer.  I will not be putting any carbon pellets in my filter.  The area that the carbon pellets normally go in will be my chamber/bowl.  Think of the carbon filter as the bottom of the GVG just made to fit in a bong.

As for the bong size, I'd like to fill the chamber with all of the vapor with one breath.  Then clear it all with a different breath.  It sounds like a 12 incher would be a little small.  Maybe I shouldn't be concerned with that though.  As long as I can get it all in one hit, I'd be fine.