It seems the ice box would be your best bet
A Peltier Effect cooler like Coleman Roadtrip 40 QT will cool to 20-25 degrees below ambient temp but they are power hogs. You must have an efficient way of charging batteries. Cost about $125. A 120 V AC dorm size refrigerator. In order to run from 12V you will need to install an inverter. An efficient refrigerator drawing less than 1 amp at 120V will draw about short of 10 amps at 12V and could be run with an 800-1000 watt inverter. An inverter of this size must be hard wired and fused. Cost including refrigerator, inverter, cables and 250 amp fuse holder and fuse would be approx. $250. The innards of the refrigerator could be adapted to your present ice box if well insulated. Portable refrigerators like Norcold, Dometic and Engel will run on 12/120V and cost between $450-700. Some run on propane. The last alternative would be be two adapt a 12V compressor/evaporator System to your existing ice box. You may have to design a tight fitting lid and add insulation. Cost between $700 and $1,200. For day sails and short overnight trips ice would be the simplest and would work fine. Hope this helps.
