Try Scale Outer and make sure that your background is bigger then the window size. That's what I used on my app and it works great. I don't think that Letterbox Scale will fill the full screen on all phones.
I'm not a expert but this what I did.
480 x 800 - after you export from Ludei this will great all screen sizes
Scale Outer - No black bars
Cocoonjs - on the ludei export I used scale to fill
My App - Sewer Fish