I noticed that a lot of models for fan grills mimic metal ones - not ideal for strength since the properties of the materials are so different.  

I couldn't find a 3D printable fan grill that I liked, so I made a few of my own.  This one has a Voronoi pattern.

It is modeled at 2mm thickness.  You can scale the Z-height in your slicer to make it thicker if needed.  I haven't tested it for noise, please comment if you have that ability.