Plasmonic phenomena can be used to realize sensors and biosensors in several application fields. This work presents polymer-based plasmonic sensor chips made by simple and cheap realization steps. The proposed sensors are small-size, highly sensitive, versatile, low-cost, and useful to realize disposable biosensors. In detail, to realize these sensors, a resin block with a custom-shaped trench inside is achieved by a 3D printer. This trench is filled with a UV-cured optical adhesive to achieve the core of a multimode optical waveguide when the end of the trench is closed by two plastic optical fiber (POF) patches. Then, a photoresist buffer layer is deposited by spinning on the optical waveguide, and finally, a gold nanofilm is sputtered to excite the plasmonic phenomenon. The buffer layer improves the sensor performance and the gold's adhesion on the surface. The optical waveguide's shape can be changed to match the sensor chip with different experimental setups, such as the one based on the smartphone's CCD camera and LED. In this way, receptor layers can be deposited on the highly sensitive gold surface to realize high-performance bio/chemical sensors for several application fields, such as Point-Of-Care Tests (POCT) and environmental monitoring.