Lots of water bottles...The best way to maintain temps while working with the tubs is to have as many water bottles in the incubator as possible.
We have water bottles up top and down below.


Do you have a fan in the incubator?
If the fan isn't strong enough then you will have higher temps up top.
If the fan is too strong you will probably have higher temps on the bottom of the inc, which is why we use a rheostat on our incubator fans.
Infinite control of fan speed allows you to fine tune the air flow so all shelves end up about the same temperature.

Pictures would help.