I used one of these, cut through the bumper with relative ease. I did cut myself, but ironically enough, it was actually the bumper that cut me, not the knife. After I cut off what was needed for clearance, I went back with a small file to smooth out any jagged edges. Probably could have been done quicker with an air saw, but I'm plenty happy with the results.
http://www.amazon.com/Snap-on-Offici.../dp/B004QO9FPC