As I mentioned in the other thread, I would go with fiberglass for the skull cap. If you are not comfortable with mixing up resin and laying fiberglass, you could try using water-activated fiberglass bandages (used like plaster bandages for making casts), which might do the trick.
For the skull cap, I would make it out of fiberglass or thick vacuum formed plastic.
For the snakes, I would use a self skinning foam and cast them over armature wire. Rigid plastic snakes might snap off on you. Also, the lighter you can make the snakes, the more comfortable it will be to wear, and the more likely it is to stay put.
I had a friend do a medusa costume that had a small MP3 player and speaker built into the head dress that was playing hissing sounds. It added a lot of style points, which always helps in costume contests.
And for some good inspiration, check out the Medusa by Steve Wang.