Oddish question: Plastic cutting.

[Mercutio]

Assuming I wanted to make a fairly precise 3.5" hole in a faceplate that was a bit too thick for a stanley knife. Best methods folks? Drill and coping saw?

Bought a dremel a while back but I can't seem to find the right disk to do a good job on the cutting.

 

[Mercutio]

Ahhh to make it clear (sorry) it's essentially a 3.5" external drive shaped hole in a plastic cover (so square). I re-read my question and it's not really that clear.

Basically a letterbox cut.