I wanted very light covers for my fans, so I've quickly designed this one. It seems not to disrupt the inner airflow at all, and prevent me to put my fingers in the blades, and protect my fans' holes when I screw too quickly.
2 versions are provided: 30x30 and 40x40.
Not the best thing in Thingiverse, but I'm happy with it. :)
Tips: As it also very thin, you can easily sanding it by applying a finger on it and move it to a sanding paper sheet.