From someone who has done MAJOR fiberglass bottom work, here's a couple of things.

1. How long was the boat in the water with that hole? If very long at all, you may want to try to borrow a moisture meter to check and see if/how wet the surrounding glass has gotten.

2. How much damage does the surrounding glass have? If it's delaminated, it will need to be ground out till you get to solid glass again. The combination of wet and damaged could lead to blisters around the area in the future.

3. Something like that, we always filled in with replacement fiberglass and resin. You say the size of a golf ball, so I would put that on borderline glass vs. bondo. IMHO, below the waterline on a mono, structural or not, anything more than just a thin layer for fairing, I'd use glass. You would do that just like any other glass project. Remove all damaged material and paint first. Overlap with the original material and put on a layer, wait, put on a layer, wait. Then once it's built back up beyond it's original level, fair back off to be even with the surrounding hull.

4. If you don't use glass, make sure to use a barrier coat before applying the bondo, want to make 100% sure that the glass is sealed since you have the center exposed. You are more likely to have water intrusion and subsequent delamination later. Believe me I've taken a sand blaster to the bottoms of boats that only look to have mild to moderate blistering, only to find that a LOT of glass is delaminated by water intrusion.