I wanted to put a semi-temporary N number in place without spending a lot of money on it. Temporary because I want someone else to paint the airplane after I've flown the initial 40 hours off it. After looking at the specifics of the regulation, which basically say the legal N numbers have to be roman capital letters 3 inches tall, two inches wide, 1/2 inch thickness with 1/2 spacing, I found my DesignCAD program worked best with Franklin Gothic Bold font, although I had to adjust aspect ratio on each letter and then line them up and space them to meet the FAA criteria. I used double sided tape to hold the "stencil" in place, I painted the lettering with black spray paint.
