So this is the starting point for the helmet. A plastic inflatable ball with a cardboard base:
Now to coat it. I’m going to use plaster-coated cloth strips that I got from the hobby store.
I took the roll of plaster cloth and cut it into strips about 2-3 inches (50-75mm) wide. The gauze/cloth strips are just dipped into warm water. The excess water is squeegeed off with your fingers, and the strip is draped over the ball and cardboard
I made a real mistake by using a few cardboard strips for the “ribs.” This left a sunken area in between the ribs, and I stuck in a few cardboard strips to try and fill it in. I think it would be much better to use more ribs or, better yet, generate a conic section that fits the shape when rolled up, and cut it out of cardboard or similar material.