I drilled a hole on each side but be careful drilling out holes use a vise or have a semi professional do it for you. To make the air holes you drill the first hole on the groove of the the 3rd rib up where if you put a pin in it it forms a 90 degrees angle to the coil. The second hole should be on the other side. Many don't drill extra holes and they are really happy. I just think the extra air flow is a nice touch especially if you are going sub ohm on you ecig. The X1 RBA is really really easy to build on. I recommend it for people like me with a couple RBAs. I highly recommend getting a X1 RBA for all vapers here for 4.50. It comes in 5 different colors to go with you ecigarette color. A step up from it is the trident clone which I think is really easy to build on two but is almost four times more expensive.
Watch out when drilling the air holes. You can get the X1 here for 4.50 . Make sure to use a vise or visit your local brick and mortar to get the extra air holes drilled.