Cooler Master MasterBox Q300L headphone holder

thingiverse

The Q300L has clip-on plastic bars in every corner that can be taken out and swapped around as neccessary for your case configuration. This thing replaces one of those bars with a custom one with mount holes. I've used it to add a headphone holder since it's pretty conveniently placed. Included is also just the mount base that clips onto the replacement bar, so you can design your own hooks, or just add a logo to your case, etc.