If you do mod an existing helmet, it needs to have the proper shape of an APH helmet with the extra width for the earcups. I once sculpted and moulded fibreglass rams horns on top of a more spherically-shaped motorcycle helmet and that did not look right.
Some details on the original APH helmets were left on when it was moulded, leaving details in the vacuu-formed helmet shells. For instance a couple of rivets and the original edge trim, which was thicker in the back of the neck. So, for accuracy's sake that should be built up around the opening's. The cast helmet shells got rubber edge trim mounted onto that shape of the original edge trim.
By the way, the Snowspeeder pilots also wore a "soft helmet" cap underneath the hard helmet. I've identified the pattern as resembling that of British 1940's RAF flying helmets. I believe it had likely been made of the same fabric as the Rebel soldier's hats: which was a medium-weight off-white cotton twill. (I've held one of those in my hands)
The chin strap was tied under the chin. (a real flying helmet would have had fasteners)