We present the development of a compact (about 1.3 × 2.0 × 20 mm3) freeform optical lightguide for sensing applications, from the conceptual design to the fabrication through injection molding. The design of the optic is based on the flow-line method from Nonimaging Optics, which allows the desired optical functionalities (45° half-acceptance and 40° beam steering) while meeting particularly tight mechanical and geometrical constraints. An extensive analysis of the effects of fabrication parameters on the performances demonstrates the importance of minimizing the fillet radius. This requisite inspired a special procedure for designing the mold, which is built as a "3D puzzle" assembly of separate pieces, each one dedicated to one specific side surface of the lightguide. This technique enables uniform optical quality on all the optic's surfaces and removes the need of a fillet radius in the mold. At present, the first lightguide prototypes have been fabricated; after the coating phase, they will be ready for the validation stage.