This paper reports the conception, design, implementation and evaluation processes of PICO, a portable audio effect system created with Pure Data and the Raspberry Pi, which augments traditional plucked string instruments such as the Brazilian Cavaquinho, the Venezuelan Cuatro, the Colombian Tiple and the Peruvian/Bolivian Charango. A fabric soft case fixed to the instrument`s body holds the PICO modules: the touchscreen, the single board computer, the sound card, the speaker system and the DC power bank. The device audio specifications arose from musicological insights about the social role of performers in their musical contexts and the instruments' playing techniques. They were taken as design challenges in the creation process of PICO`s first prototype, which was submitted to a short evaluation. Along with the construction of PICO, we reflected over the design of an interactive audio interface as a mode of research. Therefore, the paper will also discuss methodological aspects of audio hardware design.