Small patch in wall sheathing behind masonry chimney
You don't have to worry about energy loss. Your attic floor is insulated; that's the heat barrier for the home. You even want to get heat out of your attic to prevent ice dams (assuming you're in a temperate clime), but you have a vent right next to your chimney, so you'll be fine. Critters, though, could be a problem.

I don't think it matters how you cover it up; I'd put a piece of plywood on it.