On a second thought, maybe it should be even less in the center than margins. One because the center aslo gets heated plastic deposits, and also you were measuring it static, but when bed is moving the corners loose even more heat - dinamically during printing. So perhaps have the center even colder than margins.
But to print abs, its ok anyway. I have been printing in abs with perhaps even a bigger difference, for couple of years and only very big parts have warping issues. In which case i put a towel around the printer leaving on the top open. Also what does increase warping for me (a huge difference) is air draft(?) - current, e.g. if i open window or a door, especially in the winter. So i would think a chamber would help avoid this even if its not really activelly heated, its just better for air currents. I guess it still makes convection but with sort of heated air (instead of cold one), so the thermal effect is less significant.
But to print abs, its ok anyway. I have been printing in abs with perhaps even a bigger difference, for couple of years and only very big parts have warping issues. In which case i put a towel around the printer leaving on the top open. Also what does increase warping for me (a huge difference) is air draft(?) - current, e.g. if i open window or a door, especially in the winter. So i would think a chamber would help avoid this even if its not really activelly heated, its just better for air currents. I guess it still makes convection but with sort of heated air (instead of cold one), so the thermal effect is less significant.