You "might" be able to squeeze in two 18650 cells. They're 18 mm thick, and 6.5 cm long. Hm... the controls probably take up too much room.
Other than the battery thing, (which don't really matter, I'd be kinda scared to risk taking my portable into the light of day to begin with) I like it. Not sure if I posted it elsewhere, but I really like those shoulder buttons. I'm gonna try to use them as well.