You do ned at least one fan in a frezer (or any incubator for that matter), as without a fan, you wil get some striation in the freezer (i.e. warmer air high, and coler air low). As other have stated, I would use the first option. The microwave oven fans wil probably have heat produced by the motors. I had some similar fans in my original incubator design, and I could not kep my incubator below 93 degrees. I used a temp gun to determine that the motor bodies were 115 degrees. This was keeping my incubator to warm.
I hope that helps,
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