1) Get any black fan (running on 12V) from your hardware store (or old PC)
2) Pull out a
grill (bigger than the fan) from old PC or other hardware. Many many devices and equipment have fan grills or something similar; don't just think about PCs but stuff like most industrial machines, TVs, other electro equipment, whatever.
eg:
http://www.google.be/images?q=fan+guards
Instead of a real round grill you can also use a
meshed plate; this is used in even more equipment. It usually has smaller holes too than a grill. And if you don't find one on the scrapyard, you can buy this too in any decent DIY store (and often cheaper than a grill btw, at least here... and you are not limited to any size either, or shape for that matter, with mesh plate). They come in black too...
Another alternative is to use grills used for air vents in houses, airco's and machines like a washing machine or similar. eg:
http://www.google.be/images?q=outlet+grills
http://www.google.be/images?q=air+outlet+or+inlet
They come in all sorts of colors and are circular as well as square or rectangular.
3) Buy a can of black metalic spray paint (eg: car paint) and spray the grill or mesh. Do several thin layers and let it dry in between each layer. Don't spray just one thick layer.
tip: if you know someone who knows someone who knows some friendly car dealer (or any other metal workshop guy), you can opt to let the grill or mesh be powder coated together with their stuff which they want to get painted. There isn't anything better than powder coating. The grill or mesh needs to be from metal though (it needs to conduct electricity)
4) saw a hole, slightly smaller than the diameter of the fan in your desk with a figure handsaw. This one can be tricky to make it look nice and round. Take your time and don't rush it. Sand it down afterwards.
tip: make it round if you do it by hand. Strait lines are very hard to saw by hand and strait lines which aren't so strait catch the eye immediatly.
tip: If the diameter isn't too big you can usually find special '
hole sawblades' for big diameters in the hardware store; this is preferred over manually cutting the hole with a handsaw. You can also opt for more than one smaller hole for example if you don't find a hole saw that big.
Remember that the diameter of the hole will always be slightly bigger than the diameter of the hole saw. Always do a drill test first with a hole saw into a similar piece of wood (similar thickness and hardness).
And remember to always drill from the outside to the inside in wood and never force the drill.
5) mount grill on the inside with 2 or 4 small screws... Or use a strong 2-sided tape.
6) mount the fan on top of that with 4 small screws
7) connect fan to wire with molex.
etc...
All pretty easy to do actually, and cheap, but it takes some DIY and patience (eg: don't rush the painting or sawing!!) and some searching for second hand material in trashcans etc...
I don't think you'd find a 'all-in-one kit' which you can buy though. If you don't want to do the painting, you could look for black meshes or grills of course, but all the rest (like cutting the hole and the mounting) stays all the same though, you need some elbow grease for that.
PS: if the backside is open, I don't think you need to worry too much about heat though. More air circulation is always better, but I doubt it would make that of a difference.
Happy modding
----------------------------------------------------------------
EDIT: even more cheaper: simply drill small holes in a rectangular pattern in the door. No painting needed (unless on the inside of the bore holes maybe) and if the holes are small enough it will look just as good (maybe even better). Much quicker too...
Variation on this theme is to drill small holes in a thin piece of plywood (or plastic sheeting or ...). This has several advantages:
- In that way, if you fuck the drilling up or if you're not satisfied you can always try again on a new piece of plywood.
- You can cut the plywood exactly strait with a machine saw (let it cut to size in the DIY/hardware store).
- Easy to paint too with any type of paint (tip: use acrylic paint).
After that, saw a rectangular hole 1 or 2 cm smaller than this thin plate in your desk. It doesn't matter if the lines are not 100% strait because:
Mount the plate on the _outside_ of the desk with four black screws. Thus over the hole you cut in the desk. In this way, any errors or slightly bend lines will be completely covered.
And last but not least, mount a nice blue led behind it... Oh wait...
/ 
