The homogeneity of cylindrical Halbach arrays for low-field MRI is compromised by the finite length and discretisation into individual magnets. In this work we design and construct a large-bore Halbach array intended for imaging hydrocephalus in young children. The magnet is constructed using 23 double-layer Halbach rings with layer radii optimised for homogeneity. Simulated magnetic field strength and homogeneity over a 20cm spherical volume are 50.64mT and 433ppm, respectively. The homogeneity of the realised Halbach array is slightly degraded compared to simulations, but is sufficiently high to allow the use of conventional spatial encoding methods on such a system.