Methods to hide gaps underneath helmets?

Are there any methods or examples of ways to hide large gaps behind and underneath helmet builds, a little more elegantly than balaclavas? I was wanting to start an NCR veteran setup but this large gap that would be behind the helmet annoys me, and looks like it would cause the helmet to slip forward. Additionally I'd like to blend in the gaps around the jawline. I tried looking at stormtrooper helmets for inspiration but nearly all of them seem to glance over this.

So far my best guess would be to rivet in a length of 4" wide elastic to help with stability and balance, and sculpt and cast some kind of rubber panel to float above that.

Any ideas?

What about lobster plates like the A2 helmets.

Jango helmets have gaskets like this! You may be able to do something like it?

https://www.thedentedhelmet.com/for...t-skirt-gasket-with-velcro-pic-intense.18990/

Black foam padding?

Honestly, when it comes to hiding the back of the head, I always default to tall collars on the chest armor/coat. That requires your character to wear something with a collar however, so in the case your character does not, maybe an old strip of fabric like a sweat rag to drape down. Something like this on the left, but in the style of the ones on the right.



For the jaw, the only thing I can recommend that hasn't been suggested is to get that low density upholstery foam in black and line the jaw of your helmet with a piece. Angled up so its easy to get on and stays secure. However blocking off the jaw that tightly might restrict your airflow and make it uncomfortable to wear over time. But it depends if you leave spots open I suppose.