If it is a factory airborne helmet, it is molded pretty thick and a good amount of material can be removed from the inside to help with your fitting. A dremel with a cutting bit, one of the round ones, is the easiest way.
Another way if you don't have a dremel is to use small woodcarvers chisels and a hobby knife. You can find the chisels in a set in the crafting section at Walmart. I think a pack of 6 or more different ones runs $5 or so and there are two or three with rounded blades in the set.