good job, and pick up a copyright lawyer, and sell your design to a company to make microATX boxes out of it
Unfortunately Valve already has the copyright to that design. It is an item in a game, and they are already using the design in merchandising, such asthis item
Also, that might be something for the origin of this comment to take into consideration:
Funny, everyone that I have shown this to wants one. This actually took a long time to do and wasn't cheap. I would consider it if the price was right.
Making this single piece, and having not sold it doesn't violate trademark, but if you were to sell them, you would owe Valve a lot of money, and a lot more if you didn't get permission from them ahead of time.