I use an old raised bed, with a painter’s canvas drop cloth lining it, as a sandbox. The hens slowly toss all the sand out, so I refill it as needed. It seems to keep the mite populations down. They used to get clumps of mite eggs at the bases of their feathers, but I haven’t seen that in a long time now. Our soil is heavy clay, so they weren’t able to do adequate dust bathing before I created the sandbox.
On a side note, I use a litter box scoop to remove chicken poop from the sandbox as needed.
Here’s a video of my hens using their sandbox: