For some reason, I can see someone posting the pic from a couple years ago when one of the members got a branch through the firewall....
Use a step-bit, and a good one at that. Drill the smallest hole possible, and then be sure to de-burr it when you're done. Use a quality grommet, and some good RTV. I don't think that there is one location that is better than another, but judge for yourself where you can access with a drill.