I recommend Worbla pellets. http://www.mycostumes.de/Worblas-Deco-Art_1
Heat them in boiling water, pad a wig stand to match the size of your head (at least all I've found have been way smaller than any human head) and use it as a mold to shape the mass. Once it's dried, the outcome is very durable.
You'll have to be quick though, it dries very fast.