best way to mount fans in an incubator

I was just wondering what the best way yo mount a computer fan is. I always see people say to mount one top and bottom. but they never show how its mounted

I mounted my fan at the top with self tapping sheet metal screws which were long enough so there is a little space above the fan so it can pull air quietly.

Mine's mounted to the top of my incubator with long screws. I found these little cylinder spacer pieces (they actually came off the compressor when I removed it from the wine cooler), that I used between the fan and the ceiling to give it some space to pull the air.