Is there a way to create bounding boxes for elements like this, so that physics would enable the top (static) shape to capture the bottom (dynamic) shape? I've tried SVG, and shapes that are not round or rectangular don't seem to be respected.
You would need to create multiple elements and set them to be 'static' bodies and arrange them in such a way that they match the boundaries you're hoping to create. Concave bodies are unfortunately not possible so you would need to create a series of rectangles.
Here's a walkthrough that shows how to do this: https://blog.tumult.com/2015/06/01/comprehensive-physics-tutorial-for-hype-professional/
In the 'Static Element Boundaries' scene you can see how this is setup:
Thanks much Daniel.