Update:
The plaster bandages for making the hardshell didn't work out. Should've known that after an hour, when it didn't set well, something was up. The plaster was still soggy the next day so I decided to remove it; it didn't keep form at all.
Sooo, ordered some plasti-paste and did it the regular(?) way.
And then today on X-mas day, i could unwrap a great present: the mold turned out perfect!
I had to cast it to see the result: first time rotocasting went well all in all. It pooled up a bit in the front but that will be ok once I cut out the visor.
Maybe i'll try to cast a second one, a bit thinner and more uniform...
Got some paint for my airbrush in the mail today so everything ready for the next step. I coated the mold with babypowder so hopefully the primer will go on smoothly.