Kiddie pools work great. Wood can be made into a bottomless frame, too. I keep a dust bath under cover for the chickens, to keep it dry. We get a lot of rain here.
Sand makes a great base. We have heavy clay here, too. When it's not dry and hard, it's wet, mucky and takes forever to dry out. I love the sand! Mine also get in a mood for peat moss at times. Plain wood ash is a good additive, too. A little DE. Your dirt can be added into the sand, too, which will keep it loose. It's all good.