Thanks to all!
Your positive comments are very much appreciated.
It's a cardboard base with felt overlay, but as I said it was getting that damn curve to look right that was the biggest difficulty. Of course it would have been FAR easier to just go cylindrical but all the steampunk hats I've seen are curved. The method I came up with was to cut a series of crescent openings like this () in the long cardboard strip, curve it round onto the base and then tie a strong cotton round the centre to pull the sides in, which of course closes all the openings up. Be sure to initially cut the strip a little taller than you want the finished hat to be, as pulling in the sides reduces the height somewhat. Add the crown then cover with felt and decorate with the usual goggles, cogs, chains and other ephemera. It seemed to work better than I thought.
The goggles are indeed washers, (two each with green cellophane lenses sandwiched between) but in future I'd go with the thicker split washers that can be squashed flat. The rivets are just drops of PVA. As I said this hat was pretty much an experiment and I will probably remake it, or at least upgrade it for the finished figure.