If you want to use more than one shelf you will need a fan if you want the temperatures to be the same top to bottom, heat rises
The temps will be much hotter at the top of the inc if you don't.
You'll also want water bottles in there for stabilization, with every incubator that I've built water bottles were the number one best way to help keep stable temperatures.
I have 12 one gallon water bottles in my large true cooler incubator and it makes a world of difference.
Good luck with your build
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