Opol is built around an optical cage system, in which a series of parallel steel rods act as support structures for the functional components (see Fig. ). The components are fixed in place using 3D-printed plastic (polylactic acid, PLA) mounts that thread onto the rods, with a set-screw on the underside of each mount allowing the position to be locked. The two polarisers, the three beamsplitters, and the motor are held using fixed mounts; the aperture, laser and optical cell are held using laterally-adjustable centring rings; and the two mirrors are held using kinematic mounts. We describe below the function and design of each part of the mounting system, while full CAD diagrams, STL files and assembly instructions are included in the Supporting Information.