This work is focused on the design of efficient bulk acoustic resonators that can be integrated with optical microcavities towards the goal of microwave-to-optical frequency conversion. In this scheme, microwave photons are used to electromechanically excite a longitudinal mode of a high-overtone bulk acoustic resonator (HBAR) through a piezoelectric transducer. This resonance results in motion on the bottom of the HBAR, which has a mirror coating and is part of an optical Fabry-Perot cavity consisting of the HBAR and a stationary concave mirror. This mirror motion imparts sidebands on the cavity pump laser at a spacing equal to the mechanical resonance frequency, optomechanically transducing the signal from the microwave domain to the optical domain. This paper presents the design, finite element modeling, and both electrical and optical characterization of a candidate HBAR fabricated in a commercial multi-project wafer process.