First off, use a drill to make a hole large enough to fit whatever tool you end up using, I happen to like using a nibbler. You can also use a variety of saw blades-------whatever works for you. Cut a bit undersized, and finish with a file.
I would also suggest drilling small----like eighth inch holes at the corners of the cutout, and make your side cuts into these holes, finish the inner corner as needed with a file--------lots of times the eighth inch hole will be enough radius to allow parts to fit.
A small unibit is also a good tool to have here.
__________________
Mike Starkey
VAF 909
Rv-10, N210LM.
Flying as of 12/4/2010
Phase 1 done, 2/4/2011

Sold after 240+ wonderful hours of flight.
"Flying the airplane is more important than radioing your plight to a person on the ground incapable of understanding or doing anything about it."