Best solution.
Find a metalworking factory.
Ask them to cut a cookie of thick steel. Or
cast iron, if you can find it. The size of the barrel top inside the lip. 1/2 thick or more in thickness.
Cut the barrel top one inch in.
Drill several holes in the edge of the metal plate. Put a gasket on the lip, metal plate on top. Then screw with self taping screws.
Or find a barrel top, with its clamp. Same system. Again. Gasket and clamp.