One way to visualize an object's bounding box would be like this:
As Rahul mentioned in a comment, you can achieve this by adding a total of 12 cylinders at the edges of the bounding box, connecting the spheres at the corners.
If you want to render the bounding box transparently, I don't see an easy way to hide the intersections of the spheres and cylinders without knowing what program you are using. You could try to use partial spheres for the corners and see if that helps, i.e.
Maybe this is already sufficient for your purpose. (This is exactly 1/8th of a full sphere.)