Put it back together.
Its just a cap. Scrap the rubber insert. They shrink and don't seal anyway (either make your own, or do like I do and run without). The rest is a single ball bearing and a plastic cap with a small hole in it that slides on. Pop the ball bearing back in. Bit of glue around the bottom, plastic cap back on. DONE! If you've lost them I have a spare ball bearing and cap here but here is Australia and the postage would cost more than the parts depending on where you are.