Santa Cruz Array of Lenslets for Exoplanet Spectroscopy (SCALES) is an instrument being designed for direct imaging of exoplanets in the mid-infrared with the Adaptive Optics System of the W.M. Keck Observatory. The performance of SCALES will be largely affected by thermal emission from the telescope structures. Placement of a pupil stop can limit the emission of instrument structures such as primary mirror segment gaps, secondary structures, and spider arms. Here we proposed and compared three cold stops, a circular inner mask paired with circular, hexagonal, and serrated outer masks. Taking into account the pupil nutation, we modeled the throughput and the background emission for all three designs to optimize the dimensions of the cold stop.