We don't have a walk-in, but we do have a chest freezer that we put our large cooler into for freezing for any long river trips. I agree with what folks have said -- freeze in layers, use ice to pack around the food, and pour water around that 3 or 4 " at a time. Works great.
Another trick is lining the inside of the cooler with bubble wrap prior to freezing. Gives an extra layer of insulation, and a little leeway for expansion of the ice. We use hot water heater insulation -- foil bubble wrap, basically.
And, we duct tape around the lid for extra seal during transport. The duct tape doesn't come off until the cooler is needed. No biggie for short trips, but this combo of stuff has left us with ice 23 days after leaving home on an August Grand Canyon trip.