thanks Steve, your suggestion did the trick!
I was always focussed to create a good alpha channel, but always had strange looking border lines.
Do not care about the alpha channel, create markings at a transparent layer and save it as png.
I use this now just as a decal in AC3D, and can place them in different sizes and positions.
1. Create an image 2048x512 or something similar, add a background layer from a screenshot.
2. Paint markings, delete background layers, keep only the one transparent layer, save image as png.
3. Create rectangles in AC3D and use the png as texture. You can also overlay some of them.
Thomas, sorry for posting this in your airport thread. But I'm happy we finally have a solution for AC3D as well.