A falling-object protective structure (FOPS) is a safety device fitted to self-propelled agricultural vehicle that provides reasonable protection for the operator in the driving position against accidental falling objects. To be certified, a FOPS undergoes a standardised full-scale destructive test to prove its resistance to falling objects. This destructive test procedure is expensive and unsuitable for custom-made FOPS manufactured individually. In the case of a tractor in service, for which manufacturers do not offer a FOPS device, there is no certified method to evaluate the performance of a custom-designed FOPS, other than this destructive procedure. The work described in this paper, funded by the French Agriculture Ministry, proposes a design and a validation of a cantilever FOPS to equip tractors that have two-post rear roll-over protective structures (ROPS). The main requirement of the custom-designed FOPS is to meet the performance and protection level of certified structures. A design approach with the goal of achieving optimal design was adopted. At each stage, design alternatives are compared qualitatively and quantitatively, using numerical simulation, to assist decision making. A validation method based on mathematical modelling and full-scale tests, according to standardised test conditions, was adopted. This validation process made it possible to approve the design choices and to determine the scope of validity of the proposed FOPS.