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Ziplock bag has not a shape, or a mass of an egg. You are going to get false reading if your ziplock bag has a different volume that an egg (thermal mass and volume).
I would not bother with that.
Better blow an egg, flush it, fill it with water, insert a probe of a thermometer in it, then seal it.
Now you have the closest thing simulating internal egg temperature.
Anything wrong with this approach?