Bouncing Balls inside of a Movie Clip Circle

I have been playing with multiple different AS3 bouncing ball tutorials in an effort to have the balls bounce inside of a circle or other shape instead of just the stage.

Does anyone have any ideas of things to try?

I have a pretty good grip on the physics part, I am just trying to figure out how to make the balls stay inside of a certain boundary. Most of the tutes I have seen discuss top, bottom, left, and right…They do not touch on shapes that are not perfectly square.

Any ideas or direction would be greatly appreciated!
Thanks Brian