Originally, I thought reinforced epoxy was the easiest, and best solution.
Then I thought, if I were doing it, I think I would just smash the hell out
of the end until I had something shaped like a football. Then I'd drill a
very small hole through it near the end. Then I'd file out the dust cap to
suit, or smoosh until the football fit snugly in the existing opening
diagonally. Then I'd use a small safety pin for a cotter pin. As you can see,
I'm not a purist! But, if this approach turns to crap, you can still go back
to epoxy.