Buy a metal yardstick, cut a foot off of the end and cut a 12mm wide scallop deep enough to clear 70% of the nut close about a half inch from the end of one of the long sides. Voila! 12mm thin wrench!

Chances are you don't need much torque and the 12 inch length should give you plenty of leverage.

Aluminum or scrap steel stock of the appropriate thickness would work just as well, but metal yardsticks are cheap and commonly available.