The LED's convex surface is a focusing device. To diffuse the light, grind the end flat. I use this technique in the cockpit to spread the light.
The wingtip lights on a full-scale airplane are designed to be visible from straight ahead to about 120 degrees or so around the side. You shouldn't be able to see them from directly behind. The tail light is white and visible through about 180 degrees. Also legally required is a red or white flashing beacon visible in all directions laterally. A jumbo flashing LED would work for this. It can be on top or bottom of the fuselage, or both, or on the top of the fin.
Dan