Just cut out the adafruit version which basically clips together. You really want this one precision cut with a laser cutter. There is very little margin otherwise its a total pain in the ass to clip together.
If you just want something to stick the RPi in, the adafruit one isn't a bad choice. It's not as tall, perhaps a little easier to assemble. It's not something you want to be opening up all the time though and there is no access to internal pins except for the slot to the GPIO using a ribbon cable. Best thing going for it is availability.
To be fair, this is one I did cut myself using their design files on thingiverse. One you purchase from them will probably be perfect.
Built To Spec one is a bit taller, but I feel it holds the RPi more securely, is much easier to disassemble, and you can leave the top panel off to gain access to everything while still protecting the board. Light pipes are a cool idea too. I'll probably end up making one out of smoke grey acrylic and then making some light pipes out of the clear stuff.
djsb: I used my CNC machine to cut it from sheet acrylic. Probably only about $3-4 worth for each one
Edit:
http://www.thingiverse.com/thing:24461 Eh, first comment there is someone complaining that the clipping system is a bit rubbish, even on bought ones.