I was wondering if anyone could give me advice on how I could cover the back of my head and make it seem like a solid helmet around my full head?
I've actually come up with a pretty straight forward plan to make the back of the helmet out of fiberglass so it goes with the helmet front.
Hardly advice as I don't Cosplay or have any experience with it so perhaps some thinking out loud idea-tank? Reading the many Cosplay Topics and exchanged Threads, common feedback specific to tolerance is the residual heat retention and absence of breathability. Not just limited to face coverings, but in poorly ventilated Cons where herding like cattle isn't uncommon due to the number of people in attendance, in addition to the multiple layers of your matching costume you get hot. As you may know when your body is hot a reflexive cool-down is to sweat and the most common vent-point is from your head.
It may be similar to your fibreglass idea, perhaps creating a shaped casting/mould of your head (either an open-face helmet or chicken-wire skeletal frame like paper mache-ing for template) and use a similar perforated material to the green sections to continue the mask into a helmet. With obvious reasons your vision isn't obstructed because of said front mesh and as the material is continued over the top and rear you won't feel like you're in a sauna.
Thin paint-dipped aluminium is used with car bonnets for intake scoops and vented grills. Retains virtually zero heat, promotes breathability and as a soft metal, imagine it similar to a stiffened hair net?