Glen,
Whew!
That is the problem with retrofitting things, you can't always see where something else is. I just had a thought while looking at your post, in the future (not just for you but me too) when drilling through aluminum panels it might be smart to put one of those depth stops on the end of the drill allowing it to just break through enough to be able to see what is on the other side before adjusting it and completing the hole. Remember to put a rubber grommet in that hole to protect the battery cable.
Good luck.
Chees,
Jim