Are game-based Case Mods considered copyright infringement?

I've been working around with computer case mods for a while now, and I had toyed with the idea of taking commissions as a means of earning a little extra pocket money. I had a question, though. If someone commissions a WoW, Quake, or Half-Life case (to name a few), would that be considered copyright infringement? I know such designs are very popular, but I wanted to get everything straightened out before I actually open up commissions. Don't want to go to jail or anything. And if anyone can provide doentation on the laws involved, or a link to them, it would be very much appreciated. Thanks in advance!