Frigoboat aluminum evaporator plates come in two styles - flat and pre-formed into a box shape. There are various sizes of evaporators, to fit both the size of the ice box and the intended use - refrigerator, freezer or a combination of the two.
The flat plate evaporators are designed to be custom bent to fit the sides of an ice box, making that box either all refrigerator or all freezer.
The pre-formed evaporators (the H series, and B Series) are designed to provide a freezer space in a single ice box.
Some boaters use a spillover system of dividing a large ice box into two sections, with one section acting as a freezer and the second section acting as a refrigerator. A single evaporator plate is installed in the freezer section with cold air "spilling over" into the refrigerator section. A thermostat controlled spillover fan in the divider is recommended, to regulate the cold air movement from freezer to refrigerator.
For a spillover system, double the freezer space, add the refrigerator space and treat the whole box like a refrigerator when selecting the correct size evaporator.
Measure the cubic foot of your box, this need not be exact:
L" x W" x H" / 1728 (12"x12"x12"=1728", 1 cubic foot)