What are you planing to cast? If you make the whole helmet out of card board , you can brush a few coats of Laminating resin in it , this is the real name for fiber glass resin , you will also need a catalist, which is hardner that is mixed in to the resin to make it hard,, get this brush a few mixed coats on the inside alowing each coat to dry, (note if you are in a cold environment it will change the drying time so look in to the difrents temprature makes to the behavior of the mixed resin) After doing a few coats on the inside it should be hard and solid then do a resin coat on the out side , providing this has workeed for you recoat the inside then you can look in to laying some fiber glass matting , but dont do it be fore there are a few hardend coats on the interior.