...Or you could make your structures out of styrofoam and add a layer of hydraulic cement.
Simply sift it over the foam with a strainer and alternately use a spray bottle with water to wet it. You can even alter the final layer between "stucco" and "concrete" by how much water you spray during the final coat.
Sift then spray generously for a smooth cement finish or spray then sift for a textured finish. I have lots of structures (e.g., bridges, ramps) and buildings built this way and they last a very long time outdoors in the elements.
Here's what to use.