I would work on making the best patterns you can from the basic shapes first. The helmet area and the two neck areas. I would either try to vacform those parts over shapes or I would use shaped sintra. With the sintra you could first create base shapes and then bend the sintra around the shape as it was hot from boiling water. You would probably have to do the upper helmet in a few joined sections or at least two sides.
To get in and out, I would connect the helmet and neck pieces all seperately with elastic straps. That way you could stretch it on/off and likely comfortably. Still consider ventilation.
You could glass it, but I don't have a lot of experience with that yet so someone else would have to chime in.