We added a carbon plate with epoxy and one layer of fiberglass, and vacuum bagged it onto the board, this is probably a bit overkill but it holds up! If you build a board from scratch, a different approach would make sense.
But this is how we did the windsurfboard mount:
Carbon plate on the bottom. m6 threaded rods with big washers and nuts on each side of the board, these where glued and tightened before the glue was dry (not getting those out again!). Then we added some more glue/fiberglass to make sure there where no leaks. The foil is then mounted with nuts on the threaded rods coming out of the board. Not super clean, but it is super sturdy, and it will not fail. I looked at some double sided tape mount, but I do not think it will be strong enough in the long run (but I might be wrong) and I like to be on the safe side, would really suck to loose the entire propulsion unit