craftgrrl | Harley Quinn Spandex Mod

Hi-o! :D

So, for halloween my boyfriend had to be Joker, of course. So I thought I would naturally be Harley Quinn.. now my question. I decided I would modernize her, so I'm not wearing the full spandex costume, but I still do need her hat.

How can I get spandex hat to stay on my head without making it a full head piece?
i.e not down under my chin

I'll figure out how to work with the spandex but I don't know how to make it just a hat and not just a head piece. Strap maybe? But how would I hide it non-tacky style?

Thank you so much for any help or advice!