We describe a numerical model, based on a Monte Carlo algorithm, to calculate the propagation of polarized waves through highly scattering microstructured materials, and to properly account for the effect of both loss and boundaries. As an example, we investigate the impact of a strongly scattering object of air-polymer composite material on a broadband collimated source. We also calculate the depolarization of linearly polarized and circularly polarized waves escaping from the sample boundaries, especially at large scattering angles, and we show how boundaries can modify the distribution and the polarization of the scattered waves propagating out of the sample.