Look for through-hull fitting, not "vent" fitting
Since all tanks--fuel, water and holding--have the same size vent, all the "vent" fittings are "universal" and the same size (5/8"). So what's needed for a larger vent is a plain ol' through-hull...the same type your bilge pumps drain out of. They're available in a whole bunch of sizes.Btw...just got back from Annapolis last night. Great show, beautiful weather!