they might not look as refined as you'd like, but shower caps are cheap enough that you can cut mirror holes in them without feeling bad, and they look almost tailor-made for an average-sized helmet (my helmet has a visor and the shower caps fit it). i got 3 of them for $3.50 or so, just after someone gave all of our hotel shower caps to the goodwill!
the downside is that the goody shower caps i've seen only come in pink, pink, or purple.



sorry if i'm carrying on about hi-vis colors. i just want to be as safe as i can be, and not have to wear that #$*(@ ugly safety vest