The one thing that I noticed is that the widest part of the blade hook is about as wide as the largest part of the handle.
Fold it in half, with the sticky side on the outside. Then overlap the edges on one side by about 3/16 of an inch, letting them stick to each other all the way along the side.
Then take the knife handle and placed it between the two pieces of duct tape.
Wrap the duct tape tightely around the widest part of the handle, and overlap the pieces on the other side, letting them stick to each other. If the fit around the handle is not tight at this point, the blade will be loose within the sheath.
You should now have something of a tube. Take the bottom end and fold the corners. That should give you a width on the bottom of about 3/8 of an inch.
Take another piece of duct tape, 2-3 inches in length and tape it over the bottom.
Fold over the ends.
Then wrap another piece of duct tape around the circumference of the tube at the bottom end, so it covers the new piece you just taped.
Now take two long pieces of duct tape and place them over the full length of the sheath, covering all sticky areas.
Measure the length of the tube, and trim the top part so when the knife is in the sheath, about 3/4 of an inch from the handle sticks out.
You should now have a functional sheath.