I recently finished making some hills and mountains using a different type of plaster than the scupltamold stuff that you suggested. I wonder if the stuff you used is lighter than the stuff I used. The stuff I made was not quite diorama quality as I built them for my wargame table, but anyway here's a picture of some of my work on this site:
http://www.joedios.com/dioramas/sho...58&goto=next&c=
I also put up a how-to section of how I made the various scenery pieces on my site
http://thewayofwar.net/how-to