Construction of Geologically Improbable Mountain uses the conventional cardboard strip and plaster soaked paper towel method. The strips were glued with hot glue.
Hydrocal was used for the hardshell after research with many different types of plaster. It proved to be the strongest, especially when supported by a paper towel brand called "Rags" (available at paint stores). It's pretty messy and one has to work fast, but Hydrocal is cheap and provides excellent results.