The exploration of icy body composition in the solar system has often involved spectroscopic measurements of volatiles detected with remote sensing, such measurements portray materials naturally expelled from the surface that enter the exosphere and potentially escape into space. Variations in the ratio of deuterium and hydrogen in these measurements have led to inconclusive hypotheses regarding potential cometary origins of Earth’s ocean water and/or organics. Observational biases regarding unknown previous processing of the observable ejected materials necessitates studies of more dormant, less-processed bodies. Landed missions on comets have brought focus onto the development of small, sensitive instrumentation capable of similar composition measurements of the nascent surface and near-surface materials. We present an evolution of our compact Fourier-transform millimeter-wave cavity spectrometer that is tuned for sensitivity at 80.6 and 183 GHz where HDO and H₂O exhibit resonance features. We discuss both a low-SWaP (size-weight and power) architecture that uses custom microchip transceiver elements as well as a modular configuration using traditional GaAs-based millimeter-wave hardware. New design features for these systems including quartz-based coupling elements, system thermal management, and a separable clocking board are discussed in addition to sensitivity studies and applications in potential mission scenarios.