This is a 30MM cooling fan mount that I made for my Kossel 3D printer. My printer is made with 2020 Extruded aluminum, and the two horizontal pieces on the bottom are 30MM apart (center-to-center), and this is where the RAMPS board is mounted.
Just attach the fan to the mount with some appropriate length M3 screws for your fan, and the mount snaps right into place in the groves on the extruded aluminum. If more tension is needed/desired, you could put something in-between the 'fingers' to give a more snug hold, as it can be pulled out without much effort, but unless you're moving your machine around a lot, it will most likely just stay put. ;-)