Hello... figure out what kind of screws you are going to use and look for rubber " O-rings ", that will fit that screw, to place between the case wall and fan.
Typically all parts/holes with have a metric size/dimension to them... so check out the hardware store "metric screw/O-Ring/parts drawers"... You can get a lot of ideas and options that way.
1) There are methods to use a self tapping screw to screw into the Plastic fan holes... or drill and Tap a custom thread of your preference.
2) or you can use a standard hardware store machine screw and nut.
3) Simple household items like masking Tape/foam/cardboard will work as a gasket or vibration isolator.