I need to cut a single 120mm hole in a 3/4" sheet of plywood.  I've thought about buying a hole saw, but that's about $20.  I also thought about going and picking up a cheap jigsaw, and trying to cut it that way, but that might be a little rough and still more $ than I want to spend for one hole.  

Bottom line: I don't want to spend $20 to cut one damn hole.  Any suggestions I haven't thought of?