If you have an adjustable wrench and you're reasonably sure the faces are parallel, you can use a folded piece of cooking parchment paper around the nut before you clamp the wrench in place. As long as the wrench is clamped tightly onto the nut, the parchment paper will prevent marking the nut.

Take a strip about 3/4" wide and fold it in half so you have 2 thicknesses. If you leave it a little long it's easier to hold while you tighten the wrench.