That inspiration video simply used standoff bolts to screw the plastic to the monofin.  As can be seen in the end, it results in a bit floppy fin, with extra lumps, along with the future rusting of the washers.  Instead I wanted to glue it on.

I am not 100% sold on this pattern for the fluke, so I wanted it to also be removable.  The pattern I went with, pulls in, and then past the edge of the original monofin, fans out again.  This was done because the monofin has an edge made for rigidity, which I did not want to remove.  I did some swimming with the monofin unmodified, and the extra power is amazing, which I did not want to lose by making it more flexible.  My fear was that by cutting part of the edge off, I would reduce this.  So I'll try with this pattern, and if I don't like it, I can modify again later.

To do this, I hit upon the idea of using caulking as an adhesive.  I started with some silicone/latex blend that I had around:

It was easy to lay down a bead, and press the parts together, as well as to smooth the transition edges:

Once I got this all on, and dry, I remembered a problem with caulk that would make this approach fail.  Caulk cures using humidity in the air.  This means that while the transition caulking would cure fine, the bead I layed down between the plastic and the fin would never cure.  And sure enough, a day later, I pulled the plastic off, and it was still uncured.

Time for a different approach.