Cowbutt Crunchies Cosplay made a Legend of Karasuno costume, and when confronted with having to make feathers, she decided to use Worbla. She ended up having to make almost 100 total feathers for the costume. Though the finished product looks metallic, they were crafted by either sandwiching or stacking black Worbla and Worbla mesh. She recommends using the stacking method to get thinner feathers:
For my pauldrons and most of my gauntlet, these feathers are made from just two pieces of worbla pressed together. These feathers are much thinner than sandwich feathers, and you will have an easier time layering them. For some I used black worbla on one side, others I used one side black and one side mesh worbla – however for the most part this doesn’t matter a lot. Mesh worbla WILL be less prone to snapping if you are cutting particularly deep ticks into your worbla; however the double thickness of any material will make it quite stiff.
Get the full tutorial with photos at Tumblr.