Should the plate go on top or underneath? In most cases on top, because cooled air becomes heavier and sinks: air touching the plate cools, falls through the load, warms and rises again, and a circulation sets itself up in the container.
That is why the eutectic plates in GEBHARDT Isotec containers hang on rails along the top; the rails take up to 6 plates. With the plate underneath the cold air does not rise, the bottom layer is cooled more than it needs, the top layer warms, and when the lid opens the warm air that enters lands on exactly this weak zone.
The exception is heat coming from below: on a container standing on sun-heated asphalt a plate underneath makes sense too, and in narrow, deep containers a side position can work better. The rule is not to be on top but to meet the direction the heat enters from.
Where a plate touches the product, local freezing occurs at the contact point; a divider or tray goes between them. To verify, put two loggers in the same container for one trip, one at the top and one at the bottom: the gap between them shows on a single chart whether the placement works.