The tool I made took about two minutes and consisted of a narrow scrap piece of sheet aluminum. The concept is that it hooks in around the door opening and has a hole located in the same location where you can stick the tip of a pen.
It worked really well and I started by trimming about a 1/16” outside the line.
Around the corners I bumped it out to 1/8” to account for the curve and the fact that the jig doesn’t fit flush in the corners.
Another thing I did was mark the overlap of the door with a pencil on to the fuselage. I then opened to the door to see if it matched the trim mark on the door.
This was done to get a warm fuzzy that the mark on the door was close.
It’s important to keep the door closed pretty snug while moving the jig around the door. It gets difficult around the hinges but what I did was put a mixing/popsicle stick in front of the jig to help prevent binding of the jig. The other important thing is to try and keep the jig perpendicular to the edge of the door.