How do i make objects proper solid on collision? e.g. a rock hitting a metal box?

Ok so i have a rock (object) with a collider and ridge body and i have a square box made up of 4 object (sides) with colliders and ridge bodies which are fix together at the corners. So a nice solid impenetrable box.

My problem is when the rock is dropped on the box it 1. Seems to slip through into the box but if i make the collision on all objects 'continuous' it 2. pushes the box sides out of whack which spring back again letting the rock in and just looks wrong.

What i need is for the rock to hit the box, the box stays in it's form and the rock bounces off and not travel through the box objects.

What settings should i have for both rock and box objects in order for it properly be a strong box and the rock to push it but not travel through the sides of the box please?

Thanks