Lunar Flashlight is an innovative NASA CubeSat mission dedicated to mapping water ice in the permanently shadowed regions of the Moon, which may act as cold traps for volatiles. To this end, a multi- band reflectometer will be sent to orbit the Moon. This instrument consists of an optical receiver aligned with four lasers, each of which emits sequentially at a different wavelength in the near- infrared between 1 mu m and 2 mu m. The receiver measures the laser light reflected from the lunar surface; continuum/absorption band ratios are then analyzed to quantify water ice in the illuminated spot. Here, we present the current state of the optical receiver design. To optimize the optical signal-to-noise ratio, we have designed the receiver so as to maximize the laser signal collected, while minimizing the stray light reaching the detector from solar-illuminated areas of the lunar surface outside the field-of-view, taking into account the complex lunar topography. Characterization plans are also discussed. This highly mass- and volume-constrained mission will demonstrate several firsts, including being one of the first CubeSats performing science measurements beyond low Earth orbit.