I've found Vantec boxes for under $15 at NewEgg, and there are ports (you plug one end of the hard drive into the thing, it isn't enclosed, and isn't meant for continual use, just for occasional use or hot swapping). I've bought a couple in the past for $50 to $60, the one I'm picking up right now is probably going to be about $30. The box is probably the best choice, because you can use it as a backup drive without much noise or fuss, and, as John says, it is a USB plug, you don't have to get under the hood to install it.

SRS