Using the ray-optics approximation, we analyze the Casimir force in a two dimensional piston, formed by two metallic blocks sliding between parallel metallic sidewalls, which are separated from the blocks by a finite distance h. For h > 0, the ray-optics approach is not exact because diffraction effects are neglected. Nevertheless, we show that ray optics is able to qualitatively reproduce a surprising effect recently identified in an exact numerical calculation: the force between the blocks varies non-monotonically with h. In this sense, the ray-optics approach captures an essential part of the physics of multi-body interactions in this system, unlike simpler pairwise-interaction approximations such as PFA. Furthermore, by comparison to the exact numerical results, we are able to quantify the impact of diffraction on Casimir forces in this geometry.