I don’t really know what those metal things on Lightning Farron’s gloves are called, but I made them by rolling up 3mm craft foam and sticking the ends together with hot glue. I’ve also seen other cosplayers cut up empty ball-point pen tubes, though I think my way’s easier. The plate beneath them is made from 2mm craft foam. The rivets on both sides are wiggle eyes that I spray painted gold.
As always…when working with craft foam always remember to seal your foam with wood glue, mod podge or something similar before painting — and lightly sand if needed.