Hole saw and Plastic plug
I drilled a hole using a conventional hole saw, large enough to get a couple of fingers in to get at the wiring and mast bottom sheaves, with an ordinary hole saw. I think it was about a 1-3/4". The hole is covered by a white plastic snap-in plug, the kind that is used in the overhead inside boats, that I got at a local marine store. My motivation was one year when after stepping the mast I realized I had the main halyard coming out the wrong side sheave box. Before doing this, I consulted with a structural engineer at work, and he assured me that there is very little bending force at the bottom of the mast. The force at the bottom is almost pure compression, and any reasonable sized hole will have no effect on the integrity of the mast. One thing to watch out for is to not drill into the sheave boxes. They project inside the mast a fair way. My mast is an Isomat (on 1989 H30), and by drilling in the center of the flat area on the starboard side, I missed the sheave box by a safe margin.