Unusually, I've made a piece of costume for an upcoming game. This is the Star Wars Endor Trooper 'donut' helmet. It's not a 100% perfect piece of cosplay apparel but it works pretty well. I took a 3D printable model from Thingiverse, a surplus Austrian Army cold winter cap (which has suitable ear flaps), added a couple of big rubber washers for the earpieces with some 3D printed surrounds/caps and threw it together.
I don't often do 3D printed costume pieces (mostly because I hate sanding and painting 3D prints) but I'm really happy with how this came out.