If i need to do the fin i think i am going twin. I know US-box will add weight, but i will also give me some adjustment possibilities.
I think i know to put in twin-fin boxes. Make a mockup model to get the boxes alligned 13-14 cm apart with the leading edge of fin flush with front footstrapscrew, high-density foam around the box, glue it in with some epoxy and cloth, play-do in the box and glass-cloth over, and route it out afterwards. If i am wrong, please correct me.
Not hitting the sandbanks would be great, twin-fins would give me an extra 6-8 cm on them