Well, if you have access to a laser cutter, I've found the following technique quick and painless.
Technically, it is not exactly a full enclosure, just a top and bottom panel attached to each other using standoffs, but it is very versatile and easy to work with.
Step 1: Materials and Tools
Acrylic, I used 1/8" thick
Circuit Board, I used this one from Radio Shack
Standoffs, you can find these all over the place, I used McMaster-Carr
Nuts and screws that fit the standoffs, again I used McMaster
You will also need some sort of vector graphics program to create/edit the panels, I used Adobe Illustrator. Below are the files I used for this project.
The most important thing is that your panels have mounting holes in the same position as the circuit board you're using.
I've also found it very helpful to laser etch the name of the circuit into the top panel and the actual circuit diagram on the bottom panel.