MoonBEAM is a SmallSat concept capable of monitoring the entire sky for short gamma-ray transients from a cislunar orbit. A key aspect of its science instrument design is the use of phoswich technology, which couples multiple scintillating crystals to a single photosensor. This design allows it to achieve true all-sky coverage using just six detector units, with on-board flux sensitivity and localization performance similar to the Fermi Gamma-ray Burst Monitor. We will provide an overview of the instrument design as well as preliminary performance testing of the prototype detector assembly.