For support, I would try modifying a shoulder pad. Possibly with buckram. It will be very stiff, but lighter than a thick interfacing. It can be sewn and can be shaped with a steam iron. I have bought it at JoAnn's before. It's pretty cheap. $3 a yard or so.
You might be able to shape the fabric just with darts, but seeing as how there is trim that can be used to cover a seam, you might try something similar to the top part of this sleeve:
Getting the shoulder shape right will be a nice touch. Good luck!