I managed to get hold of some New Old Stock cable assemblies.
These cables have the 3 pin plug on one end and the power connector plug for the MT-1029 mounting tray (and other radio units) on the other end. The cable is 12 feet long, which should allow you to mount your Engel Fridge anywhere in the vehicle, or even in your tent!
For sale $40ea, plus post. Cheaper than the plug alone.