I think Hills average can be utilized for it but I am not sure how can I use Voigt and Reuss Models to figure out Hills average for various orientation in polycrystalline.
First question is what is the direction of mechanical loading you plan to apply: uniaxial in-plane, bi-axial in-plane, or normal to the surface? In any case, if you can perform an EBSD characterization, then you can directly plot the distributions of elastic moduli of the grains for given loading conditions and then decide how to deduce a mean value.