Actually salt has some very interesting thermal properties related to storage of energy and is being researched as a way to store heat energy from the sun and then use it to generate electricity, so it likely reduces the heat transfer from the high potential warm side to the ice. Please note that ice will be the temperature of the surroundings once at equilibrium, so the ice you make at home is usually much colder than the ice you buy and will therefore last longer.
Insulating an ice box your goal is to keep the high energy exterior from transfering to the cold interior. Also cold air sinks so you want to insulate the floor of the ice box really well and the lower sides are important adjacent to the ice. I also use the foil bubble wrap sheet on the top of my ice box and it works great. I think they used to call it a space blanket or something like that. Lastly don't open the lid a lot, keep drinks like soda and beer in a separate cooler. Freeze all your food like meat and then use it as it de-thaws. Finally you can get dry ice (frozen CO2) at WalMArt and the temperature of dry ice is about minus 100 degs. F (close enough) so it will really cool your stuff good and there is no water or mess to clean up, it just evaporates through sublimation (no liquid phase at atmospheric pressure). I can usually go a week with regular ice in my ice box, but only a couple days with coolers up in the cockpit getting opened all the time. So plan your ice as an untouchable mass for your food and a replenishable mass for your drinks and daily use.
Insulating an ice box your goal is to keep the high energy exterior from transfering to the cold interior. Also cold air sinks so you want to insulate the floor of the ice box really well and the lower sides are important adjacent to the ice. I also use the foil bubble wrap sheet on the top of my ice box and it works great. I think they used to call it a space blanket or something like that. Lastly don't open the lid a lot, keep drinks like soda and beer in a separate cooler. Freeze all your food like meat and then use it as it de-thaws. Finally you can get dry ice (frozen CO2) at WalMArt and the temperature of dry ice is about minus 100 degs. F (close enough) so it will really cool your stuff good and there is no water or mess to clean up, it just evaporates through sublimation (no liquid phase at atmospheric pressure). I can usually go a week with regular ice in my ice box, but only a couple days with coolers up in the cockpit getting opened all the time. So plan your ice as an untouchable mass for your food and a replenishable mass for your drinks and daily use.