The stuff you want is called Timtex. It is available at Fabric Depot, but I have found a slightly less stiff and thinner version at Walmart. Go to the interfacing section and feel the interfacings for the stiffest stuff they have. Timtex is the thickness of felt, but is very stiff and will hold up any stiff collar. You would also do OK with putting wire / boning into the trim area / inside the collar to act as a support to hold the shape. Just be careful about how you store the cloak because if you put a fold into the wire, it will have a permanent kink in it.
