Yeah accessories are a challenge. But your comment made me think - how about a phone case that doubles as a controller? Kind of like the Surface keyboard as a phone case or protector with tactile buttons? I found a hard case called Flitchio but not sure if it made it to market and I don't like the joystick design [1]. Here's one closer to what I was thinking [2].

[1] http://www.pocket-lint.com/news/134420-consoles-beware-flitc...

[2] http://portabletech.co/