I do packers all the time. Just wedge it in between the top grate handles, and use a little foil to protect the ends. It will flatten out a bit as it cooks, so it will turn out fine.
I haven't tried the HH method, but:
They take about 12 hours, leaving all vents wide open. I don't really worry about cooking temps, all vents wide open will be between 225-275. So at about 12 hours, it'll be pretty close to done. Check for tender with a probe, it should be about like probing room temp butter. Enjoy!