Cutting F/G cowl
Use a Dremel tool or similar.
It is best to do this outside, and use a dust mask!
Usually the air intakes have a recessed area, so no marking of the area is necessary, drill first a small hole in the area to be deburred, using the Dremel use an emery stone tool which has a conical shape, enlarge the hole made by drilling enough so you can now use an emery cilindrical accesory, these are discardable (they actually slip over an accesory with a rubber insert with a screw on one end), and the best accesory for enlargin holes in fiberglass, it will eat away on fiberglass with ease and precission.
Once you are done with the deburring, by hand now, sand the edges smooth with a 320 sandpaper wet/dry, used dry.