Some modern PC cases have two locations in which a power supply can be mounted (eg, Antec 1900). You can even put one in each if the computer requires a lot of power. Usually though, one bay ends up empty, and if it's the one near the graphics card it's a great opportunity to place a fan for additional airflow into, or out of the case.
I realised the fans form the original Xbox 360 were an almost perfect match for the PSU bay's form factor, so I set about converting them for use in a computer.
Step 1: Making the Frame
First, trim the excess plastic from the fan shrounds, as they make the fan module wider than an actual PSU (May cause compatibility issues with the back panel).
Put the label side facing down on MDF or very thick card, and trace around it. Use either scissors, a craft knife or a jigsaw depending on the material of choice, and cut out the shapes. You may need to use a drill to create a pilot hole to cut out the inner shape.
You can either sit the fans against the frame, or cut the inner shape to be slightly wider than the fans in order to sit them flush against the outside of the frame. If the frame splits in two, or frays at the corners (Particularly with soft MDF), don't worry, it is fairly easy to fix later on.
Use general purpose or hot glue to adhere the frame to the fan. build up strength between them by gluing into the corners between the frame and plastic shroud. Wood glue will be easily absorbed by fraying corners, and several coats over the entire frame will help seal any gaps and ready it for painting.
Hold it up to a PSU bay slot to check the fit, and use the holes in the PC chassis as a template to draw the hole positions onto the fan frame. Use a drill bit slightly narrower than the screw diameter, as the MDF will be soft and easily parted by the screws.
Step 2: Painting
I gave the entire frame a quick coat of matte black to help it fit with the computer's colour scheme.
Try to avoid spraying directly on the fans or motor area, although they are well protected so a small amount of paint over them shouldn't affect them much.
If the holes drilled into the frame are too wide, you can gradually make them narrower by brushing wood glue into them and allowing the MDF to absorb most of it
Step 3: Wiring
The fans together will draw about 800mA, which may be the limit of some motherboard fan headers. In this case, it is easiest to either power the fans from two separate headers, or wire them together and power them from a 12v molex cable. I attached them to a fan header, purely because I have a fan controller capable of providing sufficient power.
For reference, the 12v wires are the red and blue, and the ground wires are the black and brown ones.
I cut off the original 4-pin plug, stripped the wires, and twisted the red and blue together, then the black and brown together. After twisting the two wire pairs and soldering them to a single pair to extend them about 20cm, I used nylon cable sleeving and heat shrink tubing to improve their look and keep them tidy.