  Extra-laboratory atomic clocks are necessary for a wide array of applications
(e.g. satellite-based navigation and communication). Building upon existing
vapor cell and laser technologies, we describe an optical atomic clock,
designed around a simple and manufacturable architecture, that utilizes the
778~nm two-photon transition in rubidium and yields fractional frequency
instabilities of $3\times10^{-13}/\sqrt{\tau (s)}$ for $\tau$ from 1~s to
10000~s. We present a complete stability budget for this system and explore the
required conditions under which a fractional frequency instability of $1\times
10^{-15}$ can be maintained on long timescales. We provide precise
characterization of the leading sensitivities to external processes including
magnetic fields and fluctuations of the vapor cell temperature and 778~nm laser
power. The system is constructed primarily from commercially-available
components, an attractive feature from the standpoint of commercialization and
deployment of optical frequency standards.
