I see your problem. I would take a Sawzall and cut the top off on horizontal to the ground using the rim of the barrel as my guide. Probably would bump it with my grinder to start the hole. Once you cut the top off you would have about 1.5 inches of barrel wall between the cut and the rim. On this, I would tack some tin flashing along the edge of this with the welder or even ribbet or jbweld it in place. The lid would then be like a snuff can lid and would fit over the barrel and would keep water and other things out. Just a thought.