Can you use a metallic cover on a plastic box? The scenario I'm thinking of would be a weatherproof plastic box with GFI's and a metallic in use cover. My thinking is that there would be a bonding issue with the cover. I don't have a cover in front of me. I suppose if the GFI mounts to the cover it would be bonded. But to code? I'm unsure where to peruse in the code book to find this.