I refined the slot in the mock-up plank (sized the slot to fit the stub-mast with leather wrap) to use as a template for cutting the slot in the real king plank.
Measured, checked, rechecked, measured again… I wanted to make sure the slot was in the right position on the king plank. When I was sure I had it where I wanted it, I clamped the template to the king plank, traced the outline of the slot, cut it with the saber saw, clamped the template back on and cleaned up the edges with a router and flush trim bit.
King plank in place with the stub-mast (complete with leather wrap for sizing).
I cut a plug to fit/fill the slot and hold the mast in place. Trimmed the plug to a snug fit, which is fine for now but it will be trimmed again to allow for swelling when the plug is wet. The piece in the photo will be used as a template as there is a rather large knot in it. Even with epoxy and fiberglass, I'd rather not have that knot in the plug.
A top piece, a bit wider than the plug, will be added to the plug to prevent the plug falling through. Turn-buttons underneath will prevent the plug from popping out while sailing.