I think the problem is kind of that there is definitely no 'one mask' that will fit all people properly.
That is, even if it may go on their face, that doesn't mean it will look right on them. Figuring out the right mask shape for your face is actually really important when it comes to portraying the character, and with characters like Harley, the mask can make it or break it. c:
If you go to conventions at all, you should definitely find a booth that sells masks and figure out which shape looks best.
Or, if you super wanna make your own mask after figuring a style yourself, there's lots of tutorials I could link you to for making a mask out of leather. C:
I just personally know my last Harley mask was a total waste since, while it looks pretty on others, it looked weird on me. So I was determined to find the right mask this time LOL ; u;