First: set up your devices in a cabinet
Set up your computer in the cabinet. Wire everything.
Raise HDDs using LEGO to have a better airflow
You can add heat sinks to small metal cases. These will enlarge the surface of the case and enhance cooling. As you can see, I also put a notebook cooling pad under my Mac Mini.
Second: Modify the door (you can also modify the cabinet, but doors are easier to replace)
For the door you'll need to saw out holes for the fan and the ventilation slot
Put a dust filter over the fan hole and put the ventilation slots in
Add hinges so the door can be opened later
Make sure that the fan blows air into the cabinet, not out
Screw the fan to the door
Screw the door to the cabinet
The fan will suck in cold air, warm air can exit through the ventilation slot.
Through the vent you can also see your HDD status if you place them right. On my Lacie, blue is good, red is bad, so everything is okay
And: You're done