Although our procedures are still evolving, here's what we are doing for longer trips. We keep a 70 quart Igloo under the dinette table, covered with an old sleeping bag, with the bulk of our food in it. Then we bought a 50 quart, 5-day, Igloo Marine Cooler which is kept in the cockpit and fits somewhat nicely between the gas tanks. It's our reserve ice supply and is opened only sparingly. I made a slip-in liner for this cooler out of one- and two-inch rigid foam insulation stuck together with silicone adhesive to add extra insulation. I also made up three foam blocks which slip into the foam box. By changing the filler blocks, I can accomodate either two, three, or four ice blocks, depending on the length of trip, all surrounded with the maximum amount of insulation. We just used it on a five-day trip with three blocks and had beaucoup ice remaining to cool some trout for the trip home. For the heat of next summer, my thought is to seal the cooler with duct tape after filling it with ice to limit air infiltration, and then make a "blanket" for it, perhaps out of white beach towels, that I can repeatedly wet down to add evaporative cooling to the surface of the Igloo. I'll bet I could keep ice for ten days.
Al