A simple circuital approach to evaluate the shielding effectiveness (SE) of rectangular enclosures with apertures is proposed, considering oblique incidence and polarization. The scope of the proposed model is extended beyond 1 GHz, including higher order modes of the cavity. Furthermore, apertures are not required to be on the front face of the enclosure. In the proposed model, the SE of the enclosure with apertures on multiple sides can be simply calculated by vector decomposition. The proposed model has been successfully verified by using a conventional full-wave simulation tool. We also measured the SE of a manufactured aluminum enclosure with apertures on multiple sides.