I used gelcoat on mine but I think you can use papermache (paper with white glue).
I saw people using this method to make their LOTR armor : it is strong and cheap. The result is amazing.
Be careful with ploy urethane resins, just because they don't stink doesn't mean they're not toxic. Urethanes gas off some noxious gases when curing so always use a respirator. Aqua resin is a very good alternative and as far as I know is very safe to use.