Build your own cold box!
Keep the following in mind:
Try to create as many right angels as possible. Make the bottom as big as possible. The bottom shall be made in a size allowing the sides standing on the bottom panel. The bottom must be carefully fastened in the interior, the box filled with provisions must withstand the force during sailing in rough seas. Very often, additional supports and brackets must be manufactured. Normally it is to prefer that the top part will lie on top of the sides. The box will be more stable. The lid is cut out from the top part and can either be hinged or loosely. If the lid is made loosely, which is often to prefer, make it either absolute square or obvious rectangular to make handling easy. On top of the lid plywood or a wooden alike plate can be mounted to match the interior. The insulation end of the lid has to be protected with plastic strips, they are 3/32" thick, to create a smooth easy to clean surface. It is important that the lid is well tight, to prevent warm air entering the box. Mount a rubber sealing strip on the lid.