Finding ways to demonstrate--in a high school classroom--that subatomic particles from space produce other particles capable of reaching the Earth's surface is not a trivial task. In this paper, we describe a Geiger-Muller tube-based cosmic ray coincidence detector we produced at a total cost of less than $200, using two tubes purchased used online; if the tubes were purchased new, the total cost would be about $325. Our detector is able to produce unambiguous CR detections in just 1000 total seconds of data collection. Furthermore, it is small and easily manipulated, allowing us to easily demonstrate the relationship between cosmic ray flux and the zenith angle.