Air Vent 120mm fan clips.

thingiverse

Clips to secure a 120mm fan to your Heating/Cooling vent cover attach easily. These clips have an angled notch specifically designed to fit a fan cover like this one: http://www.thingiverse.com/thing:17028. Note that you should not print the assembly, it's simply for demonstration purposes only. I will post pictures once installed, and keep in mind these clips may not be practical in some situations as they are above the vent. After thinking about it, an under-vent version of these clips might be more useful! Maybe I'll make that next. This design was created with Tinkercad. You can edit it online at https://www.tinkercad.com/things/6I5OxhVow3M.