An important polarization enabling technology at the heart of the MSPI system is the retarder and Photo Elastic Modulator (PEM) combination, which act together as a modulating polarization rotator. The quarter wave retarders are required to perform achromatically over a larger wavelength range than provided by off-the-shelf commercial components. Meeting the design requirements for this application is challenging due to uncertainty in the birefringence models and thermal behavior of the component crystals, and fabrication limitations. AirMSPI includes an achromatic, athermalized quarter wave retarder optimized for 470, 660, and 865 nm, and demonstrates the effectiveness of a multi-crystal approach. The waveplate design of the next prototype, AirMSPI-2, is particularly challenging since it extends the wavelength range further into the blue and the SWIR. We also take into consideration that the final choice of spectral bands has not yet been determined so the waveplate must perform well over any set of bands within a broad spectral range. Our three-element multi-crystal (sapphire:MgF2:quartz) approach achieves targeted retardance of 90º± 10º when averaged over any bandpass in the 410-2185 nm range. The athermalization achieves a retardance change of less than 0.1º per 1ºC temperature change. A novel manufacturing method realizes tight retardance tolerances for the composite retarder, despite the fact that a priori crystal birefringence knowledge has considerable uncertainty, making it difficult to specify exact plate thickness values at the outset. Testing of the manufactured parts has been performed with Mueller Matrix Imaging Polarimeters (MMIPs) at the University of Arizona, and demonstrates that performance requirements for the multi-crystal retarder have been met. The result of this research is an essential step toward an advanced airborne sensor and ultimately benefits development of an instrument capable of operating in space.