When you're backpacking every ounce counts, and while most ultralighters would frown on wasting precious ounces to keep non-dehydrated food cold, but most hikers would welcome a cold beer or fresh meat after a few days of hiking. It's not difficult to construct an inexpensive and effective cooler using two thermal grocery store bags, a mylar blanket and two cold packs that will hold around the same amount of food as a lunchbox.
Instructables author Robby Skateboard created his lightweight cooler by making two pockets using thermal grocery bags and an outer cover with the mylar space blanket and duct tape. Put the food in the smaller pocket, wrap that pocket along with 1-2 cold packs in the larger pocket, and place all of that with another cold pack in the mylar outer container.
The author routine carries a half dozen eggs, a full pack of hot dogs, and two cans of soft drink in the cooler. If he freezes the hot dogs and chills the soda beforehand he's reported that his soft drink stays chilly up to a week on the trail. All for around five bucks worth of source materials and an hour's worth of time.
Backpacking Cooler [Instructables]