I had an illegal head on my Cal 20. For that boat, I cut out the fitting,
then somewhat closely matched a piece of wood to the hole. Then,
filled in the gaps with 5200, then glassed over the whole thing from
both sides. I don't know if that was the best way to do it, but it
doesn't leak and I suspect it's stronger than the rest of the hull.

Is there another method that's as or more effective?

Jonathan