How long did you let the model magic dry? To get the best durability with Model Magic, allow it to dry for at least a month. Also, if you mixed in some acrylic paint with the Model Magic before you started shaping it, the paint will give the MM a rubbery-ness after it dries, which should make it more resilient to breakage.
You could also use craft foam to make the armor. Here's a really good tutorial on how to shape the foam: http://www.entropyhouse.com/penwiper...helmsdeep.html
Hope that helps!