I just completed my Ercoupe and used a laser level to lay out the side stripe.

I set the plane on the bench with the level at the same height off the floor.

I put pencil marks at nose and tail, at the places where the top and bottom of the stripes should be.

The laser drew a straight line between these points after adjusting the levelling part.

With the line projected on the plane, I simply applied 3M fine line masking tape up to the line and started painting the stripe.

Here's how it came out: