I woul think if you just had 3 rectangles of fabric, one for the back and two for each side of the front, then you could just hem each piece (depending on what sort of fabric it is) and then attach the front pieces at the shoulder. Then sew each of the the side pieces from the bottom to about halfway up so there was a slit for arms to go through.
I made something similar out of tiger stripe fur fabric for ds so it didn't need hemming and it looked fine for a costume. You could cut out a pattern in newspaper and selotape it together so ds can try it for fit before you cut.