I have found your tips very useful, such as covering the area that you are going to cut with masking tape, but about where to cut the cowl I am in trouble here.
The fact is that I have to assemble an IMPULSE (www.bnmodels.com) and in this ARF the fuselage and the engine cowl are one single piece of fiber glass, so that you can't remove the cowl.
The engine can be mounted through a large opening, however I need to cut the cowl to let the muffler protrude off one side.
The plane deserves a neat job here and I really wonder how I will make it.