hmm Looks like you have more temperature-issues with yours than I did with mine, although you are measuring on different places than I did so maybe I just didn't discover it.Ok great! That really shows when the hatches were.
I have been playing around with it all day and still struggling to find a way to get most of it in a good temperature range. I will try put below what i have tried:
I put 4 little thermostats in the incubator in the top left, upper middle right, lower middle left and bottom right.
1. First try was with the polystyrene casing on and temperature on incubator set at 38:
TL - 36.9
UMR - 37.3
LML -35.8
BR - 36.3
2. I now took off the polystyrene casing and left it for an hour. Temp increased to 38.3:
TL - 36.3
UMR - 38.1
LML -36.8
BR - 35.6
3. I now added a hoody over the incubator for an hour. Temp set at 38.3:
TL - 36.8
UMR - 38.1
LML -36.8
BR - 36.4
4. Finally i moved the hoody so it didnt cover the UMR area. Temp set at 38.3:
TL - 36.9
UMR - 37.5 * not sure if ive mixed up this reading and the one below
LML -38.5 *
BR - 36.4
Anyway as you can see.... i still cant get all 4 areas at a similar temperature!
You've basically tried most possible combinations. What about using the casing and a hoodie to cover? That didn't work for me, but who knows.
Another possibility that I didn't try is to add a small jar with water (closed jar that is - this is NOT for adding humidity). This should stabilise the temperature throughout the bator, but it will also mean that the bator has to heat more mass so it would have to work harder... So I'm not sure how that will play out.
Another way to combat the issue is to rotate the eggs; make sure that the warmest spot doesn't stay above 37.5C (or maybe pushing towards 38C is better?), and after a few days swap the eggs around.
On the bright side; I just did an autopsy of my last 5 eggs, and to my surprise there wasn't too many chicks that had died from the temperature-issues (I had one quite at the very beginning, one in the late stages, and one close to hatch). Keep in mind it is the first week that the embryoes are most vulnerable, and that's when my temperatures differences where most severe...