I made this cape about 3 yrs ago and I'm ready to make another one. I absolutely love this pattern. I'm going to have to try adding the arm holes like you did. I made a hood with my current cape and I used magnetic buttons. I sew 1 side of the breast to the cape making the cape into a 1 way entry. So when every I put on the cape it has a dramatic effect. I love the pattern you used. It's absolutely beauty. I just bought a mustard yellow water proof fabric. I'm so excited to be water proof.
When shopping in NYC I definitely recommend Pacific Trimming for buttons and trims by the yard. SIL Thread is my go-to to stock up on zippers and thread!