You can cut and shape the foam once you are done. For covering the foam you can do one of two things. Once the foam skins over but not yet dry you can cut a slit in the foam and apply sphagnum moss or whatever your material of choice is. Or you can let the foam cure, then use Gorilla Glue. Also once the foam cures you could use a heat gun to melt the foam temporarily then apply your covering material. Lots of choices.
This is a timeline photo of this tank over the years.
Edit: Nevermind the timeline. The forums keep converting .gif into .jpg for some reason.