Okay, youve got your paper model finished(hypothetically)
Resin the outside and let that harden.
Resin the inside and let that harden.
Cut small bocks of fiberglass mat or cloth, your choice, lay them *inside* the helmet and then slather them in resin. Let that harden.
You know how a strong, fairly break resistent *insert armor piece your working on here*
Use Bondo Body Filler for smoothing the outside and use a dremel for the details.
Paint when satisfied.