Jigsaw is the best option... don't let anyone tell you different. It CAN be done with a nibbler or dremmel... bit it will be a bitch and a half compared to a jigsaw. I don't think anyone on here who has personally done the mod with all 3 tools (as I have) would say that the jig is not the best tool for the job. I'm not sure how thick your case is, but I know that it would have taken me forever to cut through my antec 1030 with a dremmel, and i'm guessing a nibbler wouldn't have been able to handle it. Besides just being easier, the jigsaw (with a fine metal blade) will make much nicer cuts. I didn't even have to sand my case after cutting, the new blade sliced right through it nice and smooth. Best of all, a jigsaw isn't that expensive. It doesn't have to be top of the line, most cheep ones will probably work fine (make sure you pick up a decent fine metal cutting blade for $2). You'll probably also fine other uses for it so it won't go to waste... Dremmel tools definitely have their place, but it's not here. Even the nibbler has many good uses... like making your hand soar... but honestly a jigsaw is what you want for this job.
Trust me... if you have some friends (which i'm hoping you do

) i'm guessing one of them can loan you their jigsaw.... they aren't that uncommon!