Good Evening, bvy,
Neither of my Chromega drums has a centering ring; that's no particular problem as long as the motor base is level.
As far as I know, the plastic used for Chromega drums is softened by the common acetone-type cement used for constructing models. If you can locate the spot of the leak, it should be simple to "melt" the plastic back together or to patch it with a small bit of similar plastic. If the bad spot is indeed on the cap, the repair could be either internal or external without affecting functionality at all.