The Hyper-Kamiokande (HK) experiment will be a next-generation water Cherenkov detector capable of measuring neutrino interactions with unprecedented statistical precision. Discriminating neutrino events from cosmic-ray muons and low-energy backgrounds is dependent upon constructing an effective Outer Detector (OD). The baseline design proposes deploying up to 10,000 3-inch high-sensitivity photomultiplier tubes each coupled to acrylic Wavelength Shifting (WLS) plates. Sophisticated optical measurements using a high-powered laser setup have improved on existing absorbance results and demonstrated a previously unknown artefact of Mie Scattering present in all candidate WLS samples. This combined with future measurements from a new test facility (Baby-K), designed to evaluate the light collection efficiency of all WLS plates via their response to cosmic muons in distilled water, will enable robust evaluation of all candidate WLS plate properties. An overview of the ongoing R&D effort to optimise the OD photosensor design, including the latest water and air-based measurements of WLS samples is discussed in this contribution.