We had a clam shell and the sand was always going moldy because water can't drain out. DH just built one last week with a few treated pine sleepers from Bunnings (about $12 each), some black plastic liner (very cheap) and 1/3m of white sand from a landscape place ($30) It's a good size for about 3 under 5 year olds.
Whatever you get, definitely make sure it can drain, and can be covered to stop cats using it at night.