Upgrade to a refridgeration unit.
Rich:Depending on what you putting in the icebox determines how you may want to load it. If you have a tray/rack in the box, you can place the block of ice on it. The cold air sinks so everything on the bottom will stay cold. Something to remember is that things on the bottom are also going to get wet. If you have things that are frozen, you can put them in a tupperware type of container and place them directly on the ice weather it be on the rack or on the bottom of the box. They tend to stay pretty frozen. We put things that we don't want to get wet in some type of container or zip lock bags. The other option to block ice is to use either milk jugs or soda bottles with lids. Freeze these in your freezer at home (without the lids on) and then just place them around the box. They serve two purposes, first as the ice that you need and then they provide drinking water as they thaw.One of the best upgrades that we did to our boat was to add refridgeration to the boat. This is not cheap but makes life much nicer. We used a Technautic Marine unit and love it. It has a holding plate and like any type of unit requires adequate battery power. So in addition to the unit that you purchase you need to add some more battery power too. Then you may want to isolate these batteries from your house bank and starting batteries (that's up to you). You should also be sure that your battery charger is up to the chore of charging 3-4 batteries.