I think this is a bit late? XD Anyway, you can pick any shirt pattern and alter it to be the shape of the cloak. You're doing the one without the left sleeve, right? Just alter the pattern connecting the left and back pieces together without the sleeve hole, and add a bit of flare to make it more flowy. Add flare to the other side, too, but keep the sleeve hole as it is. I think a pattern with a princess-seam back would look best, but you can use one without them and add darts to make it fit nicely and not just hang like a tent.
Rei the Guntecher (PSU OC)
Alphie the Demon Boy (OC)