Landscape boulders

I'm making a landscape setting for my D&D campaign. I need to make some boulders for monsters/characters to hide behind. I'm looking for advice from anyone. I tried to use plaster/paper mache but it didn't turn out very well. Has anyone done this before and if so what advice can you give me? The boulders should be a bit higher than a standard D&D figurine.

Thank you

Source: http://hirstarts.yuku.com/topic/5424/Landscape-boulders

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