@wizard69 yup, its a design that did not take control in mind, but i'm stuck with it. BTW the container is in the middle, the substrate is AL so it does conduct okay. and there is a good amount of cross coupling.
@jbb
I cant quite reveal what goes into the container, but its very sensitive to temperature, target accuracy spec is ±0.5 C.
Do you have any other temp sensors around? Maybe an ambient temperature sensor outside the oven?
I literally asked for the same thing. I was met with "lets put it on the wish list" (never).
Do you know about / have control over the heat load in the 3 inner zones?
In my initial prototype I got a stable temperatures without the outer heaters, but I only went upto 1/2 the target temperatures, due to limitations in the design. I would imagine the outer two heater play an important role at max temp.