I haven't done it yet, but I have read where guys have used a Dremel tool or a palm hacksaw.

 

I finished mine last week. I used a dremel with a cut off wheel and cut about 1/4" below the template line. Then I used a drum sander attatchment to sand it to the line. It turned out great! IMO, if you go past the line a little, it should still be OK because of how low the opening is on the bumper.