We combine design-based implementation research with a pre-registered RCT to address a long-standing challenge in psychological science: How to use psychological principles to address real-world problems while designing and implementing interventions in the field. We posit this as a design methodology for optimizing the translation between psychological science and real-world applications. We tested the efficacy of an extensively co-designed version of a game-based rational number intervention, Fraction Ball, versus “business-as-usual” math instruction and physical education in a sample of 4th/5th grade Latine students (N = 360). Insights from nine co-design sessions with 20 teachers informed revisions and additions to a previous version of Fraction Ball, strengthening impacts across 10 of 12 rational number subtests. This methodology provides insights for translating psychological science research and scaling it to address real-world educational needs, such as play-based interventions that improve rational number understanding in authentic contexts.