I don't know if you knew about this, but there are already 1K lipo batteries available, that will be 2x1000mAh, that not bad at all.
That may be the thing to do. So far, I haven't had a issue with the size of the stock batteries. It seems like it will last most of the day without any charging, but being a pass-through makes it easy to keep topped off - at least when I am at home or at work. Where do you get the 1000 mAh batteries and what model are they?
Did confirm that the center pin is adjustable. The threads don't have an interference fit, so it adjusted itself removing and installing my eGo adaptor. I noticed that my Fogger was no longer flush, so I played with the center pin and it actually has a lot of travel - 1/4" or so. I've been using mini eGo thread tanks on it mostly, so I have an adaptor installed fairly tightly on it with the center pin adjusted so that the mini tanks sit almost flush to the bottom of the well.
I'm really liking this thing. It's no taller than an eGo 650 battery, cradles in your hand better than a round battery, stands up securely when you set it down, and has tons or power compared to the other power solutions I tried. It really appears to be the perfect battery for me.