I'm getting a lot people asking off list, Asking which size to buy for 140mm size cooling fans.
You can use a 5.5" size Hole Saw Attachment. (140mm = 5.51181")
Amazon has good selections for 5.5" "Bi-Metal" Hole Saw attachment for your Power drill,
HERE
Other Hole saw attachment sizes
80mm = 3” hole saw
92mm = 3.5” hole saw
120mm = 4.5” hole saw