sander06 is spot-on with his suggestion of using epoxy - this is a job for West System and NO glass mat. In fact, the pros routinely use nylon dowels for plugging larger drill holes - first dipped in epoxy, pounded in with a nylon mallet, sanded and then sealed with a barrier coat.
This was the process the yard-workers used for draining sections of my last boat's keel, after my surveyor discovered a couple of voids.
True Blue . . .
sold the Nauticat