Dude, there's a very good reason that Chad and Rob at AltSmoke don't want you to stack batteries in their APV's. It's all over their website in RED TYPE.
The power switch is also designed to vent gases in addition to the hot spring collapsing, Chad has said nothing more is needed but if it makes you feel safer go ahead and make a hole in the battery cap.
Adding a Kick and sleeve extension is a much better idea than stacking batteries. Not only is it a safer option, you can actually adjust the voltage instead of a set 6 volts.
Super T also has the
Kicklet now that works as an attachment to the connecting pin, so if you don't wish to put up with the hassle of the Kick in the battery tube, you can put
on top of your device.
Kicklet http://www.supertmanufacturing.com/id87.html