The mast boot can be made as a truncated cone from waterproof
cloth such as Sumbrella with a double seam, or rubber such as from an old inner tube, or both; and slipped on before the mast is stepped. Failing that, since you have already stepped your mast you will have to use a wrap around boot with a folded seam in the back. The hardest thing to seal is the track extrusion that sometimes goes all the way down the mast. An extra sail slide with the bale removed that is caulked in place can help. A sealer between the boot and the mast with a large hose clamp around the boot at the mast is common. Tape over it after, to cover the screw part from catching anything. If you have a wood frame at the deck hole you can tack the bottom in place or use another clamp there. As for the mast partners wood or rubber wedges will work if they are not to severe. A more gradual wedge will drive tighter and stay in place longer. Since you have a ketch you probably don't play the backstay much, so you can block forward and aft also. If you play the backstay, the absence of fore and aft blocking allows the mast to bend more uniformly from the base. but can contribute to mast pumping (and possible failure), and loosening of the side blocking. Remember if you want to keep all the water out you have to seal inside the mast too, because water gets in at the many openings.. Generally that means that you have to drill a hole above the boot, and shoot some expandable urethane foam below the hole to completely seal the inside. Theoretically the water will flow down as far as the foam and drain out the hole.
