The complete instrument can be assembled from scratch in two days for less than $250, using only a 3D-printer and a benchtop milling machine. Opol's ease of use, compact size, fast measurement time and few millidegree angular resolution make it a capable and versatile tool for analytical science, while its low cost means it is ideally suited for use in resource-constrained environments and process monitoring. It is released here as open hardware, with technical diagrams, a full parts list, and source code for its firmware included as Supplementary Information. Beyond polarimetry, we believe the optical cage-based mounting system described here could usefully form the basis for many other types of optical instrumentation due to its ease of fabrication and excellent structural rigidity. The motor mount is formed from two 3D-printed end-plates and three 55-mm rods, and provides an expanded section of the optical cage that is large enough to accommodate the motor. The rods are secured inside pre-formed holes in the plates using M3 set screws, and the motor (blue) is fastened to one of the plates using a pair of the same button-headed M3 screws. A 1"-hole in the centre of each plate allows the two laser beams to pass. The polariser P3 is mounted to the motor using a 3D-printed ring-mount which is screwed to the motor bell using 10-mm M3 screws (see Fig. ). The mounting plate is fixed to the base plate via a short length of stiff steel wire, which also acts as a 'sprung' flexible hinge. The mirror (not shown) sits in a recess in the mounting plate, and is retained using a rubber-tipped set screw (silver). The orientation of the mounting plate can be adjusted using the two adjustment screws.