Flyerdarren,

Using a Dremel with a small grinding bit is exactly how I cut openings in cowlings.

I also mark a line on the fuselage where the cowling will be seated. Then I tape a piece of heavy paper behind the cowl attachment line. I make the engine cutout in the paper. I remove the engine and install the cowl. The paper is then used to trace an exact outline of the engine opening on the cowl. I make the cutout into the cowl and I have an exact opening perfectly matched to the engine.

Hope this helps.
John