@Joey Hess This all looks very promising.
For those of us following this wrestle with realities.......could you please summarise the current physical load (produce and water ballast) distribution and probe/s positions? Thanks.
And should you decide to add a fan, it may be worth presenting both cold and warm air to the fan (if fan is at top, conduit some cold air from the bottom, or vice versa) so that the fan is vigorously chopping and mixing the two temps.
The reason for this is that fluids of different temp tend not to mix as readily as we imagine they will:
on one job I had plumes of warm and cold air travelling down ducting, round corners etc yet remaining quite separate. This was for a big proving room, we had 6 such ducts with probes mounted through the walls giving different readings depending on position in airflow. So we introduced various baffles and spirals to tumble the flow, but with only partial improvement. The source of the problem was heating elements in the duct that did not reach fully across it (allowing air at one side to pass unheated) and we finally fixed it by narrowing the ducts so that all the air had to pass through the heating (fix the problem, not the symptom!)
In that instance, the fan was before the elements: I doubt we would have had the problem if the fan was after the elements.