You should find a discussion of this in the archives, a number of methods have been used, including building more insulation inside the box (fine if you do not mind a small box), pumping more foam it (will help, but possibly not much), and ripping it out and starting again (my solution). Owens Corning insulation, avialable at Home Depot, is the cheapest good option for new insulation, the recommended thickness is 4"-6" for refridgeration depending on your latitude, you might get away with less if it is just too keep beer in ice on a day sail.
Gareth
Freyja E35 #241 1972
