I suppose since these things cost all of $15, most people just don't bother with the DIY route.
Here's what you'd need, though: An IDE-USB2 bridge board (they are sold bare by a few companies), a USB cable, and a power supply that can output the 5V/12V that a drive needs, and also whatever power the bridge board needs.
All you'd really need to do is wire an IDE connector to the bridge board (you could either use a ribbon cable, or solder on a right-angle edge connector for a direct connection), permanently attach the USB connector to it (or not--you could just plug it in), and then wire the power supply to the bridge board and put a molex connector on it for the drive. Alternately, you could try wiring the power output of the USB cable to the bridge board so it doesn't need an external power supply, but you're still going to need to get juice to the drive.
It's probably not all that hard since bridgeboards already come with all the necessary workings and you just need to get power to them and connectors dealt with, though it'll probably also look hideous.