I'm not upset that he wants to make the costume - far from it! It's the lack of communication, and the failure to so much as ask how I felt about being copied before asking how to do so. And while I hate repeating myself, I guess I have to say this again:
When this thread was pointed out to me, my initial impression was that, instead of asking me for further, specific advice, he had instead decided to go over my head, and ask other people how to reverse-engineer my work. I hope you can see how that would be irksome, and potentially insulting.