I think how you do this will really depend on your budget and skills.
Anything from craft foam to resin to latex would potentially work here to varying degrees.
Craft foam is more "sewing" and shaping skills, whereas you could do sculpting, casting, and molding. Or you could do 3D design and fabrication.
You could even do cardboard or papier-mache or Wonderflex as a base.
There are a lot of options and it really depends on what you're skilled at or willing to learn and how much you can afford.
Disclaimer : This post written by a Grumpy Old Man
Actually in the pipes:
- Remaking Ser Loras doublet.
- Luke Skywalker, A New Hope
- Wind Waker starter costume in swim material
Eventually: Paul from Dune, Faramir, Harry Sullivan