I'd either use strips of EVA foam or even PVC foam board for rigidity and heat form it into shape, then glue the cape fabric onto it so it looks like the fabric is sticking up on it's own. You're basically just making a support structure out of a stiff, light material like this and the cape covers it. maybe?
You might try Fosshape. It's a felt-like material that hardens when you heat it. It doesn't get hard as a rock so you can still sew it after it's been hardened. You could cover the Fosshape with whatever cloth you use for the rest of the cape.
or you could look into these clear sprays that are used to stiffen fabric, just google fabric stiffening spray or something to that effect. I used something similar a long time ago on a project where i made a scale viking ship, using the spray allowed me to position the sail as if wind was blowing into it