I don't care what the
core is or where in the boat a screw is put. Drill the hole oversize and fill with thickened
epoxy then redrill for the screw. A self tapper will need a pilot hole just under the larger diameter of the screw, If the root diameter is used it will most likely be too tight and crack the filler material. I would lightly
oil the screw, with spray on
cooking oil, and screw it into the filler before it sets, obviously without the fitting it is going to hold. Undo when cured and you have a very solid mounting.
Maybe overkill but it makes me comfortable
Mike