The mast slot, in its present form, is cut into to king plank and is only one inch 'deep.'
(It will remain cut in the king plank but) I am making it 'deeper' (intending to improve lateral support when raising the mast) by adding 1" x 1½" x [the correct length] doublers on top and bottom on both sides of the slot, increasing the depth to 4". An added benefit is the bottom doublers will add some rigidity between BH1 and BH2
Also under consideration is a mast gate (no not in the plans) as a bit of "belt & suspenders" along with the mast slot plug. The mast gate will be fabricated from CVG DF. A question I have is, how should the grain be oriented?
End grain facing aft (the block shown in the photos is a 'place-holder,' not the real thing):
Edge grain facing aft:
or, Face grain facing aft? No photo but the block could be laminated face grain to face grain.