This is my version of motherboard fan cover for Ender 3. I tested many projects and they increase noise coming from fan due to flat surfaces over it which make an echo effect.
My project uses a kind of spherical shape just over the fan so the air coming from outside has smoother way to the motherboard and makes less noise. I also moved the air intake to the side which is less hearable for user compared to covers which are open in front.
You can print it without supports, there will be a little bit of overhangs inside but it won't affect on durability and usability of part, it can be easilly removed in post processing. With my settings, it takes about 2h 10min to print it.
In my opinion, this cover delivers a proper amount of air to prevent overheating but I haven't measured the temperature yet. If you are worried about it, I uploaded version with wider air intake (supports required).