I have a flycutter made to make large holes using a drill press,
General was the brand, cost something like $7 or so when I got it.
IIRC, it'll do holes up to 4" or so in thin material. I've used it in
plexiglas and aluminum, it should be run SLOW. Got it at the local
hardware store, it was sold as a flycutter for making holes, not as a
milling attachment. It's got a pilot bit that I rplaced with a piece
of drill rod, I make the hole for it first, clamp things down and have
at it. Works well enough. It's about as precise as you could set the
radius.

Stan