That is the helmet skirt the neck seal was made with a blue material not blurple cut into strips sewn together you can see the the seams the back turned around so you can see the seam going down the middle top and bottom of the material folded over sewn and had a elastic cord fed though I had a seamstress recreate it going off the gallery pics hope this helps
From what I understand, Jango did not even have a neck seal, along the lines of Boba and such. Just a high collared jumpsuit and then the blue balaclava worn underneath it.