I used an old shirt of his and some really ugly scrap fabric for the wings. I cut open the shirt, to maximize the usable area, and cut it out.
Then, I pinned it to the ugly fabric and sewed it together (leaving an opening at the top), then turned it inside out.
Since both fabrics were fairly light, there wasn’t much shape to the bat wings, so I cut out one inch wide curved pieces from cardboard that fit at the bottom between each corner. I placed them inside, then sewed alongside the cardboard to keep it in place and give the wings some shape.
I did a thick stitch from each corner to the top middle of the wings to accentuate the bat wing shape. I put on straps with velcro to hold just above the elbows and used some other scraps to make backpack-like straps.
Since he modeled my wings, I modeled his: