We propose a design method for a wideband polarisation-independent metamaterial structure by randomly mixing dielectrics. The relative permittivity (or refractive index) of the metamaterial can be varied by the volume fraction of the mixed dielectric media. The required volume fraction can be estimated by the conventional mixing formula. The designed metamaterial can be easily fabricated with a conventional 3D printer. Measurement of the relative permittivity of the proposed medium is also performed, which shows that the proposed metamaterial has polarisation independence and operates in a wide frequency band.