Some time ago, I asked how to secure a thru-hull with a flat nut to a
curved hull. Lots, of good advice..thanks to all who responded.

I tried the solution I favored, but had never tried, after much self-doubt.

In a nut-shell: I cut the top off a tupperware container..one with a stout
rounded lip. The height after cutting is about 3/8".

I duct taped it over the existing hole, top and bottom, being careful to
not distort the sides.

I then mixed epoxy with high-strength filler (West 404), until I had stiff
peanut butter consistancy, then dabbed enough around the inside of the
"mold" to fix it in position.

When that had set, I filled the rest of the mold with epoxy, and leveled
it using a 8" wide plastic "scraper". This made the top of the epoxy pad
very level...or flat may be a better word. A 2" hole saw finished the job.

Now I have a 6" thru-hull pad which is flat for the nut, and still conforms
perfectly to the hull shape.

You may ask, "How did he keep the epoxy from dripping through the
existing hole?"
Answer: I cut a 4" (or so) square of material from a freezer bag, draped
that over the large end of a tapered wood plug, and wedged it into the
hole from the outside...but not so far as to go completly through the hull...
I wanted epoxy to fill about 1/8" of the hole.

Once done, it was apparent that the doing was easier than the telling.
Norm B