There is a very large need for prosthetic components in developing countries, where such devices are imported and prohibitively expensive. This work explores the possibility of developing and manufacturing prosthetic components locally in Venezuela while preserving high quality and function. We aimed at developing a kit of plastic modular adaptors for external transtibial prostheses. The project covers design, stress analyses, and function assessment of the components. Design criteria were established from the state-of-the-art of prosthetic adaptors in commercial models and international patents. The resulting kit comprises four adaptors of simple design. Their response was studied with stress analysis, using the finite element method, applying static loads for different instants of gait during the stance phase. The simulation of the adaptors shows that the stresses presented for a person weighing up to 980 N (100 kg) do not reach the yield strength of nylon 6.6. Then, five kits of adaptors were manufactured with this thermoplastic material using conventional metal-working machines. The resulting components are lighter and cheaper than equivalent imported metallic ones. The kits were adapted to four patients and assessed via gait analysis and questionnaire. A very good function is observed, with neither significant difference in most of spatiotemporal gait parameters compared to normal values nor significant asymmetries between prosthetic and sound sides. From the questionnaire, stiffness, maneuverability, and comfort ability of the manufactured kits was found high by all the patients. A 3 months adaptation period was also completed by the patients prior to performing the gait analyses. This period is considered a first field trial of the adaptors; however, these results will be complemented in the future, as the kits were not tested to structural fatigue.