A modeling technique is presented to efficiently evaluate field penetration through small apertures within a metallic cubic enclosure. The technique utilizes the moment method solution to calculate current distribution on enclosure surface area and the modal expansion technique to find field distribution over the slot aperture. In this way, one can obtain the equivalent magnetic current along the aperture from which field distributions inside the enclosure can be computed using the cavity Green's function. Simulation results are presented to examine the accuracy and other virtues of the proposed technique.