"The first thing you need to do is study your case to find where you want to locate your blow hole. Popular spots for blow holes are on the top, front, sides and back of the case; generally near vital components. If you already
have a fan you plan on installing, you can easily test fit it to find which mounting spots work. If you do not have a fan, I suggest you make a template of the standard fan sizes to assist in measurement. Standard fan sizes are
80mm, 92mm, 120mm, and 25mm in width, although some 120mm fans may be 38mm in width."