I have never used perlite, sorry! But I wanted to tell you that I incubated my first clutch of eggs the way you are going to, in a plastic container. I measured the temps in the garage and different locations outside (at different times of the day and night) to find places where I could maintain a constant temperature. I had to move the container several times a day.
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Valerie

We may have all come on different ships, but we're in the same boat now.
--Martin Luther King, Jr.--

God bless the USA