We know that there were brass patchboxes in the 1760s. I see no problem with using one, but they would not have been in common usage in the 1760s. It was also illegal to import unworked brass to the colonies at the time too, so any brass that was available was recycled from old brass, making it even more rare. More British protectionism for English industry. If you do decide on brass, I would keep it very simple as Smart Dog suggested, but with at least some engraving. Fortunately, it is not hard to be an engraver as accomplished as most of the ancient gunsmiths... Most weren't that good, their work being little more than chicken scratches in the metal. Of course, some were exceptional.
Matt