The main issue is the hole size, not the shape. For example, if an electronic enclosure is made with a removable cover, the main criteria for emission suppression is the spacing between the screws that hold the cover. The small gap between the cover and the side walls is called a slot antenna. The length of the gap (distance between the screws) governs the frequency that the antenna will efficiently radiate. So, in similar fashion, when adding air holes to an enclosure, keep the holes small. The largest gap or hole in your enclosure dominates the effectiveness of your enclosure for suppressing electromagnetic radiation.
