The rod-and-plate cage system provides excellent structural rigidity, with the inner plates X and Y preventing unwanted shearing and twisting of the frame. The insertion of components into the cages further rigidifies the frame and, when the polarimeter is fully assembled, it forms a robust selfstanding structure that requires only a flat, stable surface to rest on. The optical cage system is laid out in three parallel lanes, each of which is formed from three rods arranged in an inverted triangle configuration. (We note that triangular cages have a slightly lower parts count than conventional square cages, and provide better access to adjustment knobs and optical elements. However, one minor disadvantage is that angled mounts acquire a "handedness", and so must be fabricated in both left-and right-handed versions.) Lane 1 contains a single 72-cm cage between Plate W and Plate Z, and holds the laser, the aperture (A), the light-to-frequency converter (LFC) and the three beam-splitters (BS1, BS2 and BS3). Lane 2 contains a short 12.5-cm cage between Plate X and Plate Y, and holds one mirror (M2) and one polariser (P2). Lane 3 comprises two cages: a 59-cm cage between end-plate W and motor-plate U that holds one mirror (M1), one polariser (P1), and the optical cell; and a 9-cm cage between motor-plate V and end-plate Z that holds no components and is needed only for structural integrity. The two motor plates are attached to the rods using grub-screws as described below.