Case modding tool help

[Mercutio]

With all the various wee case projects folks have this seems like a good spot to ask this one.

I'm after cutting a fairly precise hole in a ventilated panel (basically cutting a 3 1/2" drive bay hole in the side of a case where its got holes in/a grille for ventilation). Whats the best tool for this i'm going to be able to easilly lay hands on?
Dremel + suitable attachement or something else?

 

[Mercutio]

Hmmm, not the sort of thing I have gathering dust in a box somewhere but I guess I could ask around

Problem is though, its the small, folded side of a case so i'm not sure it would get suitably placed for a static router to get at it properly.
I was considering dremel and going a bit at a time (i.e not trying to cut a perfect 3 1/2 inch straight line first go).