Don't be surprised if your chickens don't like the sand for dusting. In your situation with them being indoor is probably fine. You may opt to take them confined outdoors and let them dust bathe there.I think I am going to move forward with making new trays that I will add sand to.
My chickens have an area of sand available,,,,,,,,,,, but never used it.
This is their preferred spot,,, my campfire pit.