Right, this is one I know the answer to, as I've done them before... Use a brass tank connector, like the one someone posted above. Don't worry about the curve. Clean the paint off where the fitting is to go, cut the hole, flux the fitting and the barrel wall, do up the fitting hard, with no seals, and the wall will form to the fitting (ie the curve is not a problem) then slacken the fitting a touch. Heat the whole lot, and soft solder, like you would do a capillary joint. I mean the nut and all (not the compression fitting, but the nut that goes against the barrel wall) The whole job then becomes not only watertight, but also physically solid, with none of the spinning round bother you get with seals. If you can't get to the inner face, you won't be able to clean it, but you will still get a bond on the front. If you can't get to the inside, tie a small weight on a bit of string. Drop the weight into the drum, and roll the drum about until it drops out the hole. Then slide the inner fitting down the string, and pull it through the hole.