The Hi-Lumi Upgrade to the LHC will utilise crab cavities to increase the peakluminosity and provide luminosity levelling at the increased crossing angle. A transverselycompact design is required to fit within the limited space between opposing beamlines. Inthis paper, further design work for a four rod TEM deflecting cavity (4RCC) is shown to besuitable for LHC. The variation of the deflecting voltage with radial offset has beenminimised by careful design. An aluminium prototype has been constructed and the bead pullmeasurements are compared to simulations. Multipacting can be a major problem for any RFcavity and simulations have been performed to ensure the shape chosen has a lower chance ofstrong multipacting, and weak multipacting can be processed through. The final niobiumcavity is expected to be held in a helium bath at 2 K, pressure variations can result indeformation of the complex shape which will alter the resonant frequency. Mechanicalsimulations have been performed to assess the sensitivity of the frequency to cryogenicpressure fluctuations, which are presented. In order to reduce the impact of these cavitieson the LHC beam, a low impedance is required for the HOMs as well as the fundamentalmonopole mode. The couplers for the 4RCC cavity have been optimised to provide effectivedamping of these modes while rejecting the operating mode.