Well, after an extensive search for a fly-cutter, I finally found one just down the street in the local hardware store. 22 bucks for the General cutter that extends to 6". Altogether it worked very well. Getting it centered exactly on the hole was slightly challenging. On the first rib I measured over and over again to find the exact center. On the second rib I just eyeballed it and would run the cutter through one or two patterns to see where it was tracking.
Following the cut I used a file and the Scotch-Brite wheel to smooth the edges of the cut.
