We use a 400 as a cooler to transport our butts/brisket/ribs. 2 bags of 7-8 lbs each of ice in a pan, I put it on the top shelf and put the meats below. Read somewhere that heat rises but cold air sinks, so put the ice on top so the cold air goes down.
I can pack it the morning of a comp or maybe the night before and usually do not need more ice. If it gets really hot at the comp, I may throw another bag or two fresh ice in there just to be safe.
Never had a problem.