I am running fans on both of my two MM ESC's to help keep them running cool. The local track tends to have a layer of small rocks/stones on it and I was worried about rocks getting stuck in the fan. So after some research I found some fan guards that fit the Novak 25mm fans I was was using. They were steel, so they would keep fingers out of harms way as well. After adding the guards I noticed the gaps would still allow small rocks to pass thru so I added a fine plastic mesh (super glued to grill). The grill can be purchased at Master Carr, part #19155K89. Just be sure to attach the guard the right way so it does not hit the fan blades.
I had to squeeze the guard a little to clear all four screws.
Plastic mesh added.