In our previous polarimeter, the functional components (optics, mechanics and electronics) accounted for around a third of the total build cost, with the remainder of the cost coming from our use of commercially-sourced optical mounts. In the new design described here all mounts and housings are made by rapid prototyping, eliminating the single largest cost factor. The components are mounted on a custom-designed frame, using a combination of milled mounting plates, steel rods, and 3D-printed fittings. In addition, we have tweaked the optical layout and made a few component substitutions to further reduce the number and cost of the functional elements. Together, the various design modifications bring the complete build-cost of the polarimeter below US$250, while maintaining the same few-millidegree accuracy as the original version. The new polarimeter -which we call The Open Polarimeter or "Opol" -can be assembled from scratch in two days or less, using no more than a 3D-printer and a benchtop milling machine. Full design details and build instructions are provided below and in the Supporting Information.