People with lower-limb amputation in low- and middle-income countries (LMICs) lack access to adequate prosthetic devices that would restore their mobility and increase their quality of life. This is largely due to the cost and durability of existing devices. Single-keel energy storage and return (ESR) prosthetic feet have recently been developed to provide improved walking benefits at an affordable cost in LMICs. These low-cost single-keel ESR feet were created using a novel design methodology, the lower leg trajectory error (LLTE) framework. The LLTE framework enables the optimization of the stiffness and geometry of a user’s prosthesis to match a target walking pattern. However, these low-cost single-keel ESR prostheses do not provide the required durability to fulfill the international standards organization (ISO) testing, which prevents their widespread use and adoption. In this work, we developed a multi-keel prosthetic foot parametric model, and extended the LLTE framework to include this multi-keel architecture and the durability requirements. This extended LLTE framework enabled the design of durable and low-cost multi-keel ESR prosthetic feet made of Nylon 6/6. Multi-keel foot designs were shown to provide 76% improved walking performance (lower LLTE values) compared with single-keel ESR designs. Load testing of prototype multi-keel feet validated the multi-keel constitutive model predictions used in the LLTE framework. The measured deflections of the prototypes under load were accurately described with an average error of 0.6 ± 0.4 mm (5.7 ± 4.2%). These multi-keel feet designed using the extended LLTE framework withstood ISO fatigue and static tests, validating their durability. Given their single-part 2D extruded geometries, multi-keel feet designed with the extended LLTE framework could be cost-effectively manufactured, providing affordable and durable high-performance prostheses that improves the mobility of LMIC users.