BEFORE ANYTHING, IMPORTANT NOTICE : DO NOT DO THIS MOD IF YOU DON'T KNOW WHAT YOUR DOING
It's a power supply, there are some pretty dangerous voltages inside, and I'm not responsible of any injure to you or someone else.
If you really want to do it, always unplug everything to the PSU and think before doing anything inside.
EDIT 16/06/2019 : I've added the original Solid Works files if you need to remix it, it's a real mess but if you can firgure it out you will be able to remix it with much more ease compared to mesh editing.
A simple to print cover for the PSU of your Ender 3, inspired by the cover of Oranginator, but I re-sketched it from scratch to better control what I can do on that part.
Not particularly optimised for airflow, it, at least, provides a good enought airflow in.
My fan is a cheap F9 from Arctic, in series with a tiny cheap DC-DC buck converter to lower the speed of the fan, and make it inaudible, works really nice !
Does not need to be printed at 100% infill, but better for structural strength of the whole thing and strong screw holes.