Wow! That sounds like a lot for a trainer to be out of balance, especially one made by Sig. Of course it's difficult to say why without actually seeing it. You can do as the other contributors mentioned, you can also add a Higley Ultra Heavy Hub. That adds 4oz. of weight out where it will be the most effective. Basic rule of thumb for balancing, fore/aft is 3 to 1. Meaning, for every 1 unit of measure added to the tail you must add 3 to the nose to balance it. I would go back over the plans and compare it to the aircraft very closely to see if you can locate the problem. Are you certain the main gear are in the proper place? It's not difficult to make some sort of mistake like that. Don't ask me how I know that either.