Letterbox scale would leave two black bars either on the sides of your game, or on the top and bottom. I'm not sure if CocoonJS supports letterbox scale you might want to contact them about that.
This is what I do, and it works well with CocoonJS.
First in the layout properties window I set Unbound scrolling to Yes. Then I make the background size much larger than the layout size like this:
<img src="http://i49.tinypic.com/2howj8j.jpg" border="0" />
After that in the project settings properties window I set Fullscreen in browser to Scale mode. Here you can see how I extended the background and sprites much farther than needed past the layout size:
<img src="http://i47.tinypic.com/muii55.jpg" border="0" />
This ensures that it will fill the screen completely no matter what resolution the device has.
Depending on the type of game you make you might have to modify these settings differently per layout. For example if you make a platformer you will want to use the settings above for the start page, and any layout that is the same size as your Window size. This will keep everything centered on your devises screen.
But for your platformer levels, layouts which are usually quite long you will want to set Unbound scrolling to NO. This will keep your platformers level starting area tight up against the left side of your devises screen filling it completely, keeping it looking nice and professional.
<img src="http://i50.tinypic.com/2db89le.png" border="0" />
I also use to find this very frustrating but after you get it working your next game will become easier to do.
This might not be a so called "real method" but it works.
I hope this helps