If the foam is held firmly in place, you could just start covering it with paper towel and your scenery base.
Yeah, Unless I slept through that part of the scenery class, foam is the modern replacement for cardboard strips, and screen, or crumpled up newspaper! It is the rough form of the final scene, which is done with plaster cloth, and hydrocal (plaster), or paper towels dipped in hydrocal. Finer detail can be added at any time after that, like ponds, rockwork, etc.. Foam buildups can also be added to change the scene as required, which are then also covered with plastercloth,or paper towels, and hydrocal.
ops: I am assuming that there is structural support under the foam.