An apparatus comprises a body that extends longitudinally between a top end and a bottom end, an elongate slot disposed in an outer surface of the body and having first and second spaced-apart detents, and a cap disposed coaxially around the body and having a cut therein that defines an elongate arm. The elongate arm has an inwardly-oriented protrusion configured to engage the detents of the slot. The cap is slidable along the body such that the inwardly-oriented protrusion slides along the slot until it detachably engages a detent, thereby retaining the cap in either a closed position that covers the bottom end or an open position in which the bottom end is exposed.