I did this project with 3 computer fans and a "Bayite" DC digital temperature controller that I found on Amazon. I leave the controller set at 75 deg. I used a piece from a plastic cutting board for the mount. This project works well and keeps the fridge 5-10 deg cooler.
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The vent cover goes over the top of the assembly in the picture. (Hope this picture isn't too big. Still trying to figure out the RV.NET Photo Posting App.)
By the way, making sure that your refrigerator install has the specified gap in the back is a good first step.