When I was planning costumes for Portus 2008, I attended a garage sale and found a couple of of iridescent purple/teal/blue 72" wide heavy taffeta. I didn't have the right trim, but I started on the gown anyway. I managed to find the perfect trim and had to sew it on at the conference. I generally travel with enough sewing supplies to almost make an outfit!
It is similar in style to my Hufflepuff robe, but the stiffness of the taffeta gives a very different look. I shorted the train to a 18" sweep. I have not yet found the perfect gown to wear under the robe.