I'd say these are probably your best bet:
http://www.3m.com/intl/ca/english/ce...cts/index.html
Find the ones that are the right size for your wreath and you should be good. The only thing I don't know is if they'll hold up if it gets too cold.
EDIT: According to their FAQs, if they're applied when it's over 10C (or you can heat the surface before applying them), they should be good.