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I've gone 12 hours, believe it or not. Heavy duty foil and a beach towel.
If you're trying to go longer than 2-3 hrs definitely preheat the cooler with hot water. You can also warm a couple bricks in the egg during the last hour of the cook and toss them in the cooler with the meat. Just wrap them in a towel so they don't melt the cooler.
Edit- Just read Whiz's page. He's got a great write-up.
"Hold my beer and watch this S##T!"
LARGE BGE DALLAS TX.