Ok heres the janky gorilla tape method rundown!
First I made a paper template of my fluke shape. I temporarily taped it to the monofin.
Then i glued the thick industrial zip ties to the monofin, radiating out almost like bones. I trimmed them to line up with the edges of my paper pattern, then removed the paper. Now i had a monofin with a zip tie skeleton attached to the end (The glue didn't stick but it doesn't matter in the long run.)
I took long strips of gorilla tape and covered the "bones" while being careful not to get it stuck to my table. Then i flipped the fin over and covered that side in tape, sticky parts of the tape together. It creates a sandwich tape, bones, tape. Then i trimmed it to shape.
uhhhh heres a picture. Yellow is monofin, blue is ziptie bones, grey is gorilla tape strips, black is where I trimmed.
I did a few layers of tape and wrapped all the edges with tape too, but this is the basic idea.
I hope this helps!
(edit: Then I put it in a fabric tail, ofc)