Scratch built is the way to go! I start with posterboard, then fiberglass it, then bondoglass the outside, sand that all smooth, then reinforce the inside with 3mm sintra. VERY strong helmet at the end. Check out my TOR mando helmet build:
http://www.thedentedhelmet.com/showthread.php?t=32511
You can use the same process on a normal Fett style helmet as well. Works great. That lid there is my 6th helmet, and I have never had any of them break or even crack. And most have seen plenty of punishment.
Good luck!
:cheers
A'den