A prototype demonstrator for the online monitoring of short–medium term radioactive waste repositories is currently under development at INFN-LNS. Such a system is planned to be distributed, fine-grained, robust, reliable, and based on low-cost components. With the main purpose of counting gamma radiation, we implemented a new kind of mini-detector based on silicon photomultipliers and scintillating fibers that behaves like a cheap scintillating Geiger–Muller counter and is suitable to be deployed in the shape of a grid around waste drums.