Small pic, but it looks like your bunks are too short. My bunks guide me to the front roller.
As far as adding a 12" roller, a good boat dealer or trailer shop should be able to fix you up. Just measure the main frame channel width so they cam match it up. If they can't help, any decent welding shop can bend you some brackets to fit a 12" on your trailer.
It's the Bill of Rights, not the Bill of Needs.