Do you run them on the ESC
BEC? I see your fan pulls 0.66 A, but I also see a lot of server fans that pull 1 A. Either way it seems quite hard on the ESC. Have you noticed the ESC get hotter? Could also run them on their own battery, or on an external
BEC or UBEC, but in my eyes that's just more points of failure and ofcourse it all costs $$$.
I seriously need to get my hands on some window screen, my current cover isn't fine enough. I have one suggestion, if you put some sort of spacer on the sides of the fan, the mesh gets suspended above the fan. I have noticed that often grass get sucked onto the fan cover, and it doesn't go all the way through the fan, but the ends get sucked below the cover and just touch the top of the fan blades. Hard to explain this but hope you get what I mean. I think this was the main cause of my fan dying. You could use some longer bolts with a stand-off (or just a nut) under the screw head. Ofcourse you could also use a chassis shroud as
@Horatio often suggests

.
I am also curious about the durability of the server fans. I see lots of people use them, but they surely aren't built for all the vibrations and shock in a RC. But then, I also doubt the cheap rocket fans and all the rebranded version are built much different.