In this paper we present a series of design directions for the development of affordable, compliant, modular, underactuated robot fingers, that can be used as prostheses by amputees that suffer from various partial hand amputations (index to pinky fingers are considered). Our design is based on parametric models that have been derived from hand anthropometry studies. Various interfaces have been considered in order to control the prosthesis, depending on the type and level of amputation. More precisely: 1) An Electromyography (EMG) based interface is used to control the robot fingers employing the EMG signals of the human forearm muscles 2) A flex sensors based interface is used to record the motion of the intact finger/fingers and predict the motion of the prosthesis implementing a synergistic behavior in an efficient manner, 3) A body powered interface is used for those that want to achieve even lower cost, with robust intuitive operation. Following the proposed design directions, an amputee will be able to replicate our fingers and develop personalized, affordable, light-weight but yet efficient prostheses.