I'm sure that the professional costume designers know exactly how to make it work and look just like the iconic hood. My guess is that there would be some kind of "under-mask" worn by the actor, like with the Spider-Man mask, and then the actual mask is mounted onto that to ensure that it hugs the right spots and looks baggy in the others. Also, the mask would probably have the be made in a similar way to a ski mask, as you suggested. There are a lot of cosplay Cobra Commanders out there that really suck because they essentially made their masks the way you'd make a cheap ghost costume out of a sheet for Halloween. It's just a big sheet of fabric draped over the person's head, so it looks incredibly stupid. A professional costume maker, or even a very skilled cosplayer could get it right without too much fuss.