For the dust bath, a mix of peat moss and wood ash is GREAT....they love it....I have the same sandbox setup.....do your chickens get a bit scared when you take off the top of the sandbox?
Reviews by Mayakovsky
rosemarythyme
Oops just saw this comment - nope they don't get upset by the top coming off or on the sandbox. They look forward to the sandbox being opened up especially in the winter, when it's the only dry part of the run, so they gather around when I open it.