Alright, I take it your not doing this for a living so I will give you a GREAT tip (its a secret!). You don't actually need a hole saw. Actually, thats probably the most cost-INeffective way to do it. Those things, as 120mm sizes are EXPENSIVE. Instead, buy a 6-dollar tool that looks like this (honestly, I cant remember what it is called).
Show this to any hardware store and tell them what you wanna do and you should be set. And this should save you about 30 bucks. Also, use that saved time and money to concentrate on centering your blowhole, as it will look SUPER silly if you mess it up. One last thing, start on the OUTSIDE with this device, because when you break through to the other side it sometimes makes rough edges. However, I did this with my case and it came out looking PERFECT. Good LUCK!