A mirror would be best, but would be pretty fragile. So I was thinking a piece of highly polished aluminum, glued to the battery cover with a couple holes drilled in it for the cover screws. I've seen mirrors in rest areas made out of that stuff, but they're not as reflective as the glass ones.
I wasn't going to go out of my way to do this right now, but I was going to keep my eyes open for some material like that. If anyone finds this kind of stuff in the meantime, please let us know.