This subject was the subject of a recent conversation on this site. I have used ice cubes and gel-packs of all kinds, but the best way to maintain cold in your chest is to freeze fresh water or juice in 1 or 2 litre plastic soda bottles. It will thaw out over 24 to 36 hours depending on conditions. When melted you also have good drinking water. I find that the 2 litre size will last me a whole weekend. It is like having a block of ice without having melting water making a mess in the chest! The best way to improve the performance of your built-in ice chest is to remove the plywood around the well and use spray foam (polyurethene)insulation found in any building supply store.