Hi,
I've only built a couple of meter yachts, but knowing of this problem, but maybe not of use to you if you can't now get to the box,? I do the following.
I set the keel box into position with a couple of spot tacks of glue. I then make a frame round the outside of the box, and epoxy the frame to the hull, leaving 1mm between the two items. Using 1/8 by 1/4 balsa for the frame. Once the epoxy sets, I remove the keel box and then epoxy back into place. After this has set I apply epoxy to the frame and box area letting it run into the 1mm gap. I use the very slow setting epoxy for the last bit, to give it a chance to get to all the little areas that might have leaked
Bob