Just get several internal batteries. The 1st battery slot is hot-swappable. Just make sure the second one never runs out (swap after 5 hrs, then every 3 hrs thereafter). Better yet, just stop and swap both, surely there's a 30 second gap in a 6+ hr recording session.
And do not use an external 6V battery. It says 5V and it means it. I read that it chokes hard at 5.6V, and can be damaged.