Since there are a lot of sharp corners and the horns, I suggest looking into the "Rondo" method to coat the inside of the helm. Rondo is just a mixture of resin and regular Bondo body putty. It can be mixed at different ratios to get a runny or thick liquid that can be used to coat your piece. It allows you to get thicker coats of material in or on your project, giving you a very strong, sturdy finished piece without the need for fiberglass cloth. Its great for small intricate details and hard to reach places, like inside solid objects.
However, rondo has to drawbacks. First, its weight. Cloth is harder to get into small places effectively, but is generally a good bit lighter. Rondo allows you to get into details, but it often goes on thick, so it weighs more. The other issue is that it shrinks a little as it cures. If your piece isn't properly enforced, it has the chance to warp what you are working on. The ways I've seen to combat this is simply to apply at least two coats of fiberglass resin to the outside instead of one.
I use this method a lot instead of working with cloth, and its usually a lot quicker than dealing with the cloth. Plus its much less of a head ache in the end because you are a lot less likely to leave a hollow spot in your armor. And the only time its ever failed me is when I was working on an assault rifle. I didn't properly support it and it warped beyond repair. I did it twice. To two guns. Because I don't learn lessons.
I hope I'm not rambling. I just like to give advice.