You know Ebb pictures of what you have done sure would be helpful.

But I get what you are talking about now. According to the manual the original designed width of the sheeve slot in the mast was 13/16" but like yours mine was not that wide. I'm using two Harken halyard sheeves model number 6062 which are 1-1/2" diameter and 11/16" thick. If I use 16 ga. S.S. for the sides of the box assembly my total thickness should be just under the 13/16" designed width of the mast slot. That would be a goal for me since the head stay and the back stay are connected above this entire area. And depending on the design of the box assembly the way it is attached to the mast could also add additional strength to the top of the mast.

If I get a chance this weekend I think I will spend some time with my CAD software and see what I come up with.