In recent years, the impressive growth of the sports automotive sector has intensified the need for enhanced pilot safety, prompting the enforcement of increasingly stringent regulations, ensured through homologation tests. Passive safety systems, such as crash boxes, undergo quasi-static and dynamic homologation tests to assess structural integrity and energy absorption capabilities. This study focuses on the design of a composite rear crash box for a racing car under dynamic homologation requirements. From a sustainability perspective, the work proposes replacing carbon fibers with natural reinforcements, i.e., flax fibers. Natural fibers are promising solutions for reducing the carbon footprint of designed components. However, their mechanical properties are not comparable to those of synthetic counterparts. In this regard, inter-ply hybridization of carbon and flax fibers is considered. This strategy aims to combine the superior mechanical properties of carbon plies to minimize performance degradation, with the reduced environmental impact of flax ones. Experimentally, the work investigates the axial crush testing of flat and circular tubular coupons for energy absorption characterization, deriving curvature-dependent crush stress parameters. Using these coupon parameters, an explicit Finite Element (FE) model of the rear-crash box is developed in Abaqus/Explicit, coupled with the CZone add-on, to predict the dynamic response of the structural component and support its certification process, applying curvature-dependent data to model the different geometric zones of the FE structure. The adopted modeling strategy produces results that compare favorably with those obtained from testing a full-scale crash structure. To obtain the optimal flax–carbon hybrid configuration, an optimization procedure based on a genetic algorithm is implemented to ensure crashworthiness certification while maximizing the carbon footprint reduction.