Anything that stays on the head, I imagine. I am quite aware of how random and poorly supported both practices are. However, if one feels a religious imperative to cover one's head- and it's such a strong minhag (besides all that Shulhan Arukh business that no one quite listens to, confusingly enough) that it does seem to be important, then why should it matter what one uses for the purpose? Especially if there is no way that any particular head covering would identify one's communal/religious leanings accurately...