Without knowing your specific masthead arrangement it's kinda hard to recommend a solution, but I personally have never seen a masthead sealed at the top,(doesn't mean it don't exist
maybe in mast furling
?) seems impossible with internal halyards, and most sheave boxes are open on the bottom. But if you want to avoid drilling holes thru the casting, perhaps route
the cables thru the side of the mast just below the casting with a grommet to protect them from chafe and a clam vent covering the hole would work to deflect most water
intrusion. That is how my VHF
and masthead transducer cables are routed. Or maybe something like this- CableClams - Blue Sea Systems
might be what you're looking for if you have the room up there.