If you're looking for off-the shelf, and don't want to build one, a couple of suggestions: ATM products and Cool Components at one of my favorite web retailers. The cool components cabinet vent fans are an easy way to get the air in & out.

Another thought: I have a USB laptop cooler inverted over my DVR, which has 3 USB ports. Works great.

However, like Alphanstein, I prefer to DIY the fans. I recently started using USB, which carries 5v and up to 500ma, to power some vent-mounted 60mm 12v fans in my cabinet (2 pushing, 2 pulling), USB via the x360. The lower voltage makes them run slower, but much quieter, and they run constantly.

My hat's off, Alphanstein, to your innovative solution.