You will never get a perfect line cutting it out of ultrakote or monokote. An easier way is to go to the auto paint store (not auto parts - auto PAINT) and get vinyl tape in the color you want and the thickness. If they don't have it, they can order just about any color you want. Then lay it down on the plane - as you get to the curve, work slowly and stretch it as you go and it will make a very nice curve without wrinkles. If you can't find them at the auto parts store, check with some of the vinyl cutters (I got some from Dennis Kirby at Kirbys Kustom). They are called "end cuts" because they are cut off of the end of the vinyl roll. The auto PAINT store will have a broader variety in both colors and sizes though.

Hope this helps.