Step 3: Install PC fan into cooler
This is a 'forced air' type incubator, which mostly means there won't be cold and hot spots inside the incubator. You don't need much of a fan to accomplish this: a simple PC core fan will do the job nicely.
I have used a hot glue gun to attach square legos to the back side of my fan, as spacers, so the fan isn't smack up against the wall of the cooler. You might use a bit of the leftover styrofoam from the lid, if you like, instead. For this fan, I have tried attaching it to the wall with bits of wire hanger. I find this allows a lot of vibration and thus more noise than necessary, but I have not yet found a tape that will stick to styrofoam, so there you go. You can experiment with how to best attach the fan to the wall. I like to attach mine so it blows across/above the light bulb, moving the warmed air.
Cut the jack off the end of the power cords on the fan. Poke a hole through the wall of the cooler, just large enough to run the wires through to the outside. Strip the ends of the red and black wires, about 1/2".
For power, you need a 12V transformer. Any old 12V 'wall wart' transformer will do: I have used one from a long-defunct baby monitor, and one from a phone charger. Cut the jack off and separate the two halves of the wire (You will end up with black and black with white stripe, or similar). Separate the two a few and strip the ends about 1/2".
Plain black goes to red; black with white stripe, goes to black. Yellow is ground and gets nothing, although you may wish to just put a bit of electrical tape over the end anyway. Twist the stripped wires together and cover with wire nuts or with electrical tape. Plug in and test. It should run smoothly and near-silently.