Foil works well for heat retention, you can also help keep the heat in by pre-warming your cooler (to make it into a hotter instead, joke). I either pour a couple of gallons of hot water in and leave it for ten to fifteen minutes, or I fill two gallon water containers with hot water and put one at each end of my large cooler, then put three towels on the bottom of the cooler and place the foiled meat on top of the towels, then another three towels on top of the meat.
I have had temperatures still in the 160° ish mark three and a half to four hours later when it is time to serve.
I am not a great believer in plastic wrap (sounds strange I know), but I don't believe that it is "good" for you, when that close to hot (still cooking) food.