Never done what you are asking, but it may save you some dough... Rather than custom build one...
Check out tigerdirect or Newegg for an empty micro case. They have front-USB's, and include their own power supplies. They are fairly small, and there would be ample room for cooling. Some even have cool lights!!!
I currently have 3 320gb hdd's sitting in a storage server that I'd temporarily like to convert into a small external USB 2.0 device. Instead of separate multiple enclosures (and separate power cords and usb cables) for each, I'd like to combine it all into one. Here's what I'm thinking:
Use expansion hdd kit to mount the drives. Fan on the front will help keep them cool.
Use this interface from Addonics - lets you connect 4 IDE drives into one USB connection, gives the option of showing each drive individually (which is what I'll use) or together.
To power the drives, interface, and fan, just get a small PSU. Anyone have an idea what wattage I should shoot for? Something like the one below, perhaps. I don't want an external brick, but I want the internal part to be as small and compact as possible so that the overall device can be as small as possible. I've seen some PSUs meant for use in carputers, but aren't really familiar with them.
The big question is what do I put all this stuff in? I want the case to be basically big enough to hold all this stuff, and with a handle on top. Anyone know of custom case manus I can give specs to who'll cut a case? I don't have the tools to cut and bend metal. How do I wrap all this stuff up into a neat package?