Here's a cheap alternative... Stick the computer back in it's box & cut some holes in it. Just watch the temperature.

Those kits look cheap enough, I'd look in to them. They should be safe, easy to install, and they're more likely to work than something home grown.

Also, if you can find a way to de-couple the fan from the case - tiny rubber bands for example, the case probably acts like a sound-board for the vibrations the fan makes. I've noticed that a fan that's not attached to a case is pretty quiet, and gets noisy once you've attached it to the case.

Also, I've seen these absorbant pads you put under the computer (in case it piddles). No, seriously it's supposed to de-couple the computer from the floor. A couple of old mouse pads should do nicely here. Try picking up your computer and seeing how noisy it is v. being on the floor, or whatever it's on. Maybe some rubber feet might do the trick.

Good luck to you.