Accurate and time-effective simulation and design optimization of quasi-optical (QO) systems is a challenging electromagnetic problem given the multi-scale dimension of QO components and the need to consider the finite size of such components to account for effects like diffraction. To show that the Method of Moments (MoM) provides an elegant solution for this, truncated rectangular arrays of tilted slots are measured in a QO THz time-domain setup and comparison with MoM is carried out. The extraordinary transmission peaks are modulated by the size of the array and the orientation of the slots with respect to the incident E-field.