How big is your cutout? How thick is your board? It seems to me like
a jigsaw would be a good way to go. If you don't have one, maybe you
can drill more pilot holes and chisel out between them. It seems like
making the entire cut would be pretty hard on your router and bit
unless you make a rough cut to close dimensions first. Start with a
jigsaw or scrollsaw, just inside your line, and clean it up with your
router using your template?