Steve,
I hope this doesn't make a mountain out of a molehill, but here goes...
In many peoples' opinions, 'exploratory surgery' is always required on fiberglass damage. IMO, it's the most important step. There is no way to know the extent of the damage until all cracks in the damage have been removed through poking, prodding, prying, grinding, etc... causing the least additional damage possible.... until nothing remains but undamaged original material.
Follow Zeke's most expert advice. He was the original owner of Ensign Spars, was the class builder for years, and obviously 'knows his stuff'.
I have a small offering which might help. Building a quick, positive mold is something I would consider and it might further minimize the clean-up and help shape the repair. Depending on the need after 'exploratory surgery' a positive mold can be made using some stiff, crunchy foam or even WetFoam, both of which are cheap and found at Michaels craft stores. WetFoam is not actually wet when you buy it, it's just named that because peeps use it to create flower arrangements and it does hold water very well. When you buy it, it is quite dry, and quite formable just with your hands or even by 'jamming' it into any particular shape for really accurate conformity. There is an article on pedestal repair on the ECA website that uses this technique:
I'm imagining two pieces of foam to form your mold. The first is produced by simply jamming a block of this foam into the foot of your mast. (Might need to poke a hole through the block first and insert a backed-up extraction cord for removal.) The second is formed by pressing a piece of foam into the recess on the ceiling inside the cabin. Put the two together with toothpicks, wrap it in cling wrap for a release agent, and voila, you have a really quick and easy mold for your work.
Tape off the area on the ceiling to catch drips and runs, and catch the epoxy when it is soft-cured to easily cut runs with an exacto blade to reduce or eliminate sanding.
Best of luck!