[backyardchef]

Anything you can do to add to the thermal mass at the the temperature range you want to hold the briskets at will help-- for instance a couple of firebricks wrapped in foil heated to 180 or so will help keep the environment toasty for the beef longer. I usually fill any air gaps with newspaper, towels or an old sweatshirt. The less empty space in the cooler or holding box-type thing, the better.