You know you could have just used Hydrospan. Its the exact opposite of Hydro shrink.
Thats a Product that is a casting resin but reacts when water is mixed. Hydro shrink hardens then shrinks as water is evaporated from the Cast. Hydrospan is the exact opposite, put it in a bucket or bathtub and as it absorbs water it will grow uniformly, resulting in a larger version of the original. I really am surprised noone here has used it on this helmet.
We guys in the Action figure customizing realm love this stuff. This way we can even shrink 12" figs down to size. OR even those Action Clix Halo figurines up to 3 and 3/4th inches (the standard action figure size) We then dremel them out onto Micromen figures(japanese figures with the best articulation around 27 points I think).