make a skull mask and wear it inside your helmet? you'll have to scale the helmet accordingly to fit it inside it.
if you wanted to take it a step further and really go wild you could even do something like this : Bones Foam Latex Prosthetic Appliance
I've bought from that company before and their products are incredibly well made. it will substantially increase the amount of time it takes to put the cosplay on, and they're not as reusable as the rest of the costume but you would be SUBSTANTIALLY more comfortable if you had good ventilation inside your helmet versus having a mask or something 3d printed to stick on the inside. Not to mention you can make facial expressions and freak people out.
just a thought though. the process for learning how to apply, dress seams, do makeup etc can be daunting but fun.