If you were making a "replica" helmet, what material would you use?

aluminum foil cuz you can do it fast

apoxie clay is strong. make a helmet out of tin foil and put the mixed up apoxie sculpt on it to strengthen it. it does not need baked and is very hard when cured. takes 24 hours to cure. use something to support it as it cures.

mix part a with part b in equal amounts before applying.

it comes in many colours and is amazing.

yes -- natural clay is good, but it does have a fragility that makes the material a bit useless. Personally, I use paper -- it can hold the structure quite well, and looks a lot better and more natural.

Have you thought about EVA foam? Cosplayers use EVA foam for all sorts of props and costumes because it’s flexible, easy to bend and manipulate, and accepts all manner of paint. For inspiration look up Bill Doran of Punished Props on YouTube (here’s a link for foam types: https://www.youtube.com/watch?v=rqZ2EBAGzz8) and you might also try his books Foamsmith, Foamsmith 2, and Foamsmith 3.

I'd use metal or a combination of leather and metal.

Tin foil.              

just try all that and see, it might work

There's art clay that you can bake, as well as mold and sculpt before you bake it. Paintable after baking.