If your helmet is a heavy rubber, it’s probably going to be covered by medium armour. There’s a specific ban on foam armour, but if I was making vegan ‘leather’ Medium Armour, I’d totally have a go with rubber flooring. I’ve seen rubber car mats used in sci fi armour before.
I am not a ref, but if I was buying something in a cast rubber I’d want it to be on the “norton armour” side of production values and not the “fancy dress shop one”. If it’s trying to look realistic and doesn’t flop about like an empty rubber mask, I can’t see how it’s different from stiff leather or syntheric urethane rubbers.
From the Medium Armour Rules
Thick leather armour (the majority of the armour is more than 3mm thick) or any armour made from a light weight material such as polyurethane (e.g. Norton Armour), aluminium, or mixed neoprene and steel provides the wearer with 3 extra hits.
At which point, if you’re wearing it it will count for the purposes of extra hits. The rules benefit of helmets is you just need one of those and some chest armour and you have full coverage. So if it’ll count as medium, you’ll have +3 hits if you wear it with a Medium or Heavy breastplate, for example.