In order to achieve an efficiency that will make this work, the box
insulation is of utmost importance. 'Polyurethane' foam is not a
likely candidate, rather *extruded polystyrene* is an economical and
well-suited choice. Available at some (most?) Big Box home
improvement stores as either the Dow 'blueboard' or the Owens pink
'Foamular', Ex-*truded* P. is highly resistant to water absorption, an
absolute prerequisite to any insulating foam in the marine
environment. Ex-*panded* polystyrene, sold as dow 'Styrofoam' - often
white, and comprised of what appears to be small balls compressed
together - should be avoided.

If you're building your own box, and can seal the insulation
completely, polyisocyanurate (also available at the 'home' stores) is
an even better choice (higher 'R' value) - but it will absorb moisture
if not *completely* sealed.

See Glacier Bay's informative site on marine refrigeration, including
their study on insulation:

http://www.glacierbay.com/Library.asp

MW