Last Saturday my Hasty Bake was up about 300 degrees for a couple hours while cooking, and the brisket still came out exceptional. So some movement in temperature won't hurt a thing. That's one thing I've had to get through my head over the years. I used to worry when the temperature moved 5 degrees.
I always use a water pan under my brisket. Partly to keep some moisture, partly as an insulator from the fire source.