Metric drill bits are not readily available in the US, but Grainger carries them in their catalog.
The hinges were clamped to a straight edge for alignment
The root cap was removed with a hacksaw at 90-deg to the hinge barrel
The foam was removed to a depth of 13mm. Some flox was used to smoothen the gouges at the bottom
Trial fitting the nutplate
After trial fitting the nutplate, the foam was glassed, and edges trimmed. Holes were drilled for the anchor nuts. Nutplate was a little too large to fit inside the cavity. It has to be milled along its sides. Holes were drilled for the tails of the anchor nuts and their rivets.
Anchor nuts were riveted to the nutplate. Modeling clay was stuffed inside the anchor nuts to prevent epoxy from getting in. The nutplate was floxed in place and covered with glass. Holes were drilled through the glass to access the anchor nuts.
The washers were a little too large to align with lower right hole, so they had to be shaved off. Rudder horn was attached. Screws were not tightened because the horn may have to be removed in the future before final installation.
The sharp curve at the leading edge caused the laminations to lift, leaving two voids below the surface. A few holes were drilled, and flox was injected into the large void; pure epoxy was injected into the smaller one.
Completed rudder