How do I center the game window with cocoonjs on iphone

hello!

my game is designed for the latest iphone 5 screen aspect ratio in portrait mode. I export with Cocoonjs. Id like the game to have a letter box screen when viewed with other aspect ratios, and have black bars on either side, however, the game window will appear all the way to the left with a black bar on the right instead of centering when viewing on other aspect ratios....any thoughts?

thanks

Dan