If the box is just one geometry, the nthe collider will be convex, and the physics engine does not “see” that there are walls. It’s just a solid block.
You need to explode/slice the box so that the walls are separate features from the bottom of the box, and then add a collider to the root feature. This way, the box should actually appear as a box in the physics engine.
It’s currently quite difficult to understand and visualize the collider shapes, so we hope to improve this over the next year.