I've seen period artwork that showed the mittens as having a leather palm and a "slit" down them so that the hand could be slid out. I think all of those were attached to a mail sleeve, so I'm not sure how relevant that would be to what you're putting together. If you aren't doing mail cuffs, it should be quick and easy.
I got my mail riviting pliers from Jeffrey Hedgecock at Historic Enterprises, and my rings and rivits from Duke Icefalcon; though the rings have been on backorder about 3 months now.