Just a thought here.
I'm thinking it may be easier to just sew one. That is if you have a sewing machine. There is also a product out there that you just iron to make seams and to attach other fabric pieces. That would eliminate the need for a machine anyways. I used that for a curtain for a little closet and it worked like a charm.
Too bad, I used to have a cape from my boys when they were smaller. They used to run around the house and jump off the couch to "fly". Goof balls!
Hmmm, I should look once, it may still be around. It was from a Halloween costume that my boys outgrew.