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Obviously insulation is a key factor. The more and better the insulation, the less energy required to keep a unit cold. A good design will have four to six inches of extruded polystyrene, such as the [http://www.dow.com/ Dow chemical] Blueboard (STYROFOAM) or the [http://www.owenscorning.com/ Owens Corning] InsulPINK Foam Insulation Board, plus radiant barrier insulation (RBI).
Polystyrene is water repellent, meaning it does not absorb water, which gives better performance in a moist environment. For the tropics, [http://www.glacierbay.com/ Glacier Bay] recommends consider R20 in a refrigerator and R30 in a freezer, which corresponds to four to six inches of foam. RBI has not been tested in a fridge/freezer, but it is inexpensive to add in a custom build so worth the chance as an experimental outermost layer.
Separating the fridge and freezer units is another good idea. You can optimise each unit without compromising or spilling air from both when you only need access to one. Fridges are usually kept at 4.4 degrees C (40 F), while freezers are kept at -6.6 C (20 F). Keep the freezer as small as possible.
