I hate cutting metal. I want to add the fan mod to my tek fixture, but I'm dreading the cutting.
I've modded a number of computer cases. For bigger cuts, I've used a combination of the fiberglass reinforced cutting wheels (not the carbide, diamond, or toothed ones) and a jig saw with a blade for metal cuts. If you go slow and use tape, you can get a decent cut with the jig saw.
Probably goes without saying, but safety glasses are a must for those cutting wheels. Even if you take regular breaks, a couple of them heat up and break over the course of a long cut. Earplugs are also nice for cutting metal with a jig saw (so loud).