In my dirt run I use dirt for them to dust bathe in. They dig a hole wherever they want to so that's in their control, not mine. It's not always a convenient location but it makes them happy. When it is dry that dirt makes a really nice dust for them to dust bathe in.
Yea, I provide the container in the covered run for when it's too wet out in the yard. When the yard is dry they prefer to dig their own holes. When it's wet they all use the one I provide.
JT