I just finished building a 22 long filter cartridge. The inside tube is four inched and the outside is a little less than eight leaving about 1 3/4 inch space between them for the carbon. The tubes consist of a double layer of hardware cloth (1/4 inch square mesh) with aluminum window screen sandwiched between them. I calculate that itll take 3 1/2 gallons to fill it. Activated carbon isnt cheap but I am. Has anyone ever tried filling a filter with cat litter to see if it is effective at removing odors? Ill probably try it anyways, if it works Ill let you know.