If you really want to do it yourself, and you're able to do basic sewing, go to Wal-mart or some place that carries fabric and look for Simplicity Design # 5840
and make a couple of changes:
If possible, exagerate the hood (make it longer, wider...all that really has to follow the design is where it attached to the cloak),
Also make the sleave cuffs larger. A sleeve is basically a triangle with the sides sewn together to make a cone. Leave the end that attaches to the cloak the same, but make the cuff end longer, and you have wide cuffs)
and if you'd like, sew up the front seam (or add hooks or buttons).