Randomly Populating Shapes with MovieClips

I can place the movieclips randomly on the stage but I am trying to randomly populate shapes with movieclips.

ball_mc.x = Math.random() * stage.stageWidth;

I tried just replacing stage.stageWidth with box_mc.width but that didnt work. Also, I wanted to create more complicated shapes for their bounding area then just a movieclip’s rectangle.