There is no doubt as to that being a lta. yes they will sometimes attach to sand, but isn't the goal of our home aquariums to try and replicate the animals natural envoirment to the best of our knowledge? If you look around, there will never be a picture of a long tentacle anemone in the rocks. Sand Only.
I'm basically echoing what svxh6 says, the container trick will work and will make a happy lta.