I've made a Superman cape and a dragon's hood with a collar thing so this is what I would do:
Cut rectangle from duvet to size including the neck to leg, and the collar times two (you want to fold over the collar and sew in the stand-up supporting material.
Fold over collar area and sew to give a pocket to put in the stand-up supporting material and a thin pocket to put a bit of ribbon or elastic for a tie.
Now....use your imagination that the below is the material......
__(fold)
^
(double thickness, wrong side together) (know this width from ^^so you
cut right size side stand up
material)
^
----------(sew)
(thread elastic/ribbon/curtain cord through here)
----------(sew)
_(edge of material)
_(end of cape)
put the cord length through the thin pocket. Pull to gather material together. Possibly best to tack it to the cape so that it stays gathered.
Once gathered slot in your stand-up supporting material - I would use some thin plastic (scrap plastic sheet, something laminated?) - or could use cardboard...but think plastic gives better shape. If using plastic make sure corners are rounded so won't poke out and jab Dracula) - cut out the width and length (dependent on how much its gathered). Fold over edges to inside and sew to finish off collar.
Hope that makes some sense!!!!