Bill of Material:
1) 120mm PC Case Fan
2) PC Case Fan Screws
To power your 12V fan with just USB juice, buy this...
USB 5V to 12V 2.1x5.5mm DC step up converter
Best source for these would be ebay.
Note: PC case fan uses distinct self-tapping screws, ordinary ones might not fit properly.
Assembly is quite self-explanatory:
1) A base, which you can either have as a stand or wall mount
2) A set of arms with grooves, to support the mounted fan
3) A set of fan brackets that provide the ratcheting action
4) A fan guard, front and back are same, if you prefer so.
Print with largest surface on laid flat heat bed to ensure good printablility.