Plaster that cracks usually means it either wasn't supported right or it dried to fast IME. Plaster if it is under any kind of tension will crack, it may not like if you just put the plaster on the foam. I would either use plaster soaked bags or other kinds of paper (if you can get old gauze bandage cheaply, that works great too), and use that as a base. If it is really warm where you are working, spritzing it with water can get it to cure slower (the same technique is used with concrete to make it cure slower and prevent cracking). Once that sets, you could then use plaster to create more of a landscape, and again while that is curing spritz it to slow down the curing.
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