Hello all! I would like to build a small 2' X 2' X 8" box to place inside the coop to give the girls a place to dust bathe. Can someone give suggestions as to what to add to this box? I have fine play sand, hardwood sawdust from my woodworking shop and hardwood ashes from the woodstove. Although, I think the ashes probably shouldn't be used. Maybe just use a bag of black dirt? Please reply. I hope this is the right place to post this. Thanks!