The Shadow Box concept is not constrained to rectangles. I use variations of Shadow Boxes in every conceivable way, always with the goal of adding depth and visual dynamics to the slides. Here are two samples I created that use a circle and triangle.
Here I duplicated the circle, removed the outline/stroke and went into the Fill Effects exactly the same as described in the previous post. But here I am using a color for the center and I manually distorted the circle to give it the cast shadow effect.
Here I duplicated the Pyramid, removed the outline/stroke and went into the fill effects. The difference is that I set the Shading Styles to be Horizontal so the gradient went from the top of the image to nothing at the bottom. I then really distorted the autoshape (extending it far beyond the boundary of the viewable slide) and positioned to create the drop shadow effect.
Troy @ TLC