ironbot:
http://i46.tinypic.com/4ghrmd.png
I've only just looked at your sketch. That's not what I meant at all. The additional piece needs to be higher up and turned through ninety degrees so that when the 'bot pivots around the axle, the ends of your additional piece move a long way. This is what gives it the inertia about the axis.
Responding to your later post, could you explain what signal your sensors give you and how you get the 'bot's lean angle from them? Are you actually working out the lean angle at all? Given an accelerometer at the top of the 'bot, you might only be detecting for/aft acceleration; that wouldrespond when the 'bot tips rotates around its axle, but will also respond to acceleration from your motor as you try to balance the 'bot. That's going to need more analysis to separate the two signals if that's what you're hoping to do. So, how are you determining the lean angle of your 'bot?