http://www.alleycatscratch.com/lotr/makingem/Patterns/Cloaks.htmThat is a really neat site, I haven't tried any of the patterns yet because I don't have a sewing machine and it really wasn't a task that I wanted to tackle by hand. But I think you might find what you need there, you can eaisly alter the pattern to look how you want, the great thing about making a cloak/robe is that it is all slack fabric and there really isn't much but stragit lines to sew. You can also find all sorts of places online that sell all kinds of clasps. If you need anymore info or anything lemme know and I will be happy to help.
3nodding And as for the shirt, I dunno where you live or what your doing for hallloween but, here its cold as hell during Halloween so I would recomend a long sleeve shirt.