I just started working on the same project! I have always done masks in leather, and some LARPer friends of mine asked about getting something made. Leather, unfortunately, goes flat if it gets too wet, so I thought it wouldn't be good for outside gaming.
So I've started to play with craft foam. Turns out you can heat mold it, so if you warm it with a curling iron or just over an electric range, it will go floppy and then hold whatever shape you put it in. It isn't strong in terms of resisting tearing, but you can whack it with blunt objects all day long and it will bounce right back.
I don't have any techniques perfected yet, but I'm basing my initial experiments on a great tutorial here:
http://www.entropyhouse.com/penwiper/costumes/helmsdeep.html
on making foam armor reps.
Good luck, and tell me how it goes!
Sarah